The reason why soda pop cans are cylinders has three main reasons; it lacks the weak corners of a cube, has the same silhouette as a cube and a sphere depending whether you’re looking from the side or above, and its manufacturing hits a sweet spot between the two aforementioned shapes. For a mecha design, while a recognisable silhouette is important, and the thematic motif whether or not you want your look robot to look industrial war machine or hand crafted master piece, you should keep your setting in mind whether or not to use cubic or cylindrical shapes.
It is much cheaper to build an object that is shaped like a cube. It is relatively fast, cheap and easy, and by standard your cube’s can deviate few degree. However, no matter what material a cube is made of, its weakest spot will always be the corners. You can have it on its flat side, but if you were to hit its corner, all the force from the impact would be on that tip. The corners, while not as weak, too are weak spots, as they are thinner in material than the rest of the cube. When metal is bent, it stretches. The stretching is dependent on the material and the angle. A very rough rule of thumb is that steel 1mm thick has to be 1mm shorther than its intended measures, as the material will stretch 1mm during bending. This stretching naturally wears and stresses the metal down, and it now being thinner, is weaker. This can be countered by having additional material welded into the corners inside the cube, but this extends production time and adds mass.
Of course you could weld the material together by its corners, but that would introduce heat and additional material. As the rule of thumb goes, when you heat metal up, it hardens, tempers. This might introduce a weak point. This is why you often see the weld seam to be in the middle of a side, because then the corners won’t become tempered by the heat. Then again, welding in itself is kind of intentional mistake whenever it is done. It would always be best to have any and all pieces as one whole, but manufacturing such objects is often extremely expensive and nigh impossible. Mechanical and chemical bonding are also good options to consider, e.g. airplanes’ surfaces are nowadays glued together with epoxy adhesive rather than riveted.
Cylindrical shapes on the other hand don’t have similar weak spots as such, as the whole thing can rolled from one sheet. A robot can weld a large cylinder together rather quickly from its ends. Nevertheless, by standard creating a cylinder shape is somewhat more expensive to manufacture than a cube due to the sheer amount of control the rollers must be under at all times. Unless the process has been automated to a high degree, and the general shape is simple to manufacture in proper fast steps, producing a cylinder can become stupidly expensive. The history of soda can design has allowed the shape and the manufacturing to be extremely efficient and automated, something that is not probably possible in a mecha setting for giant robot limbs and body shapes.
While G1 animation largely ignored the whole issue of corners, the fact is that most mecha designs realise that having hard corners is not a good idea and camfer them down. It might add complexity to the work scheduling and design, but its a small price compared to the benefits
No fiction really concentrates on these things really. Its all about the visual flavour of the design. Mazinger Z has a body based on round torso and cylinder limbs, because that’s what Go Nagai went for in a comic that was quick to work on. Gundam in general uses flat, cubic surfaces because that is industrially more sensible to work with. If you look at the difference between The Federation and Zeon Mobile Suits during One Year War or, you should see that, in rough terms, Zeon has a high number of machines with round and bulbous shapes in their MS, which all are rather expensive to manufacture, while Fed’s mass produced units like the GM are very much made cheaply with cubic shapes governing the limbs and body. Knowing OYW’s setting and its technological level, no MS production is fully automated. Building just one MS takes hundreds of people, of which part simply work to shape the sheets into their proper shapes before a welder prepares the parts for a welding robot to make the final seams, before another person comes in to check how the welding robot has fucked up this time and how he has to fix. We can safely assume that one of the many reasons why GM’s production cycle is much shorter than Zaku II’s due to the geometry of its armouring.
Another benefit in GM’s overall shape and silhouette is that it has a smaller profile than the Zaku II. Much like with real world tanks, the GM seems to have balance between mobility and armouring. It has less mass to move around compared to the Zaku II, though the Zeon MS most likely has better armouring overall. Nevertheless, its silhouette has more room of error what to hit. However, in a world where you have particle weapons able to pierce armouring like its was butter, GM’s silhouette offers less a target to hit. While MS designs would grow in size with time, and get all kinds of shapes to them, these considerations really aren’t an integral part of Gundam‘s base design idea. After all, in real world all Gundam designs have to adhere to the fact that the franchise is model kit driven first and foremost. This dictates everything from colours base faction design differences.
Tanks are the best real world example of balance between silhouette and best of both worlds with shapes.
The above PLA’s Type 99a has a showcase of three important bits; round shapes, flat shapes and angles. The design of the turret tapers at the front in order to direct bullets and other hits away from the front section. The same applies to the front of the tank, where a hit in an angle has higher chances to bounce off. If you look at the GM above, its flat surface invite bullets to penetrate it. The humanoid form is not exactly the most bullet resisting shape, hence why some mecha design elongate the chest further forwards to create a sort of cone effect. The silhouette of the hull increases in size towards back for the same reason as the turret. While its side view might be rather flat, there’s nothing much that can be done there outside additional armouring, be reactive or just additional plating. You want to face your opponent face to face to offer them the smallest possible target with the best protection you can offer. Certain tanks can also lower their aim below the horizon, meaning they can stay behind a hill and offer even smaller target.
While tank battles and their function is not directly relatable to giant robots, a Realistic setting would take into notion the cost of designing and producing shapes that make a mecha. It’s alright to want to design a super sports car for more unrealistic setting, where cost of production is no issue, but a serious take on mecha warfare is required to recognise that shape must not only be dictated by use and utility, but also by material and cost. Tanks give some idea for good utility of industrially probable shapes, while fighter jets can be used as reference point for aerodynamics.
Makes you wonder how the hell did Kouji’s grandfather manage to build Mazinger Z in his basement.
How does all this affect the design you might have in your mind? Depending on your angle you take within the fiction, the overall governing surfaces are dictated by the intention. If they’re intended to be slow walking tanks, consider what direction is intended to be against the enemy fire and add proper slants or curved surfaces. Even if the angles around GM’s cockpit are laughably small, the fact that they are there gives some idea where the design was going. Sure, in reality it probably was just to conform to the humanoid shape, but we can assume the angles were calculated somewhat to encourage ricocheting. Zaku II armour seems to have designed under notion of heavy armour to withstand firepower overall, though as mentioned, particle beam weapons don’t really care unless you have a repelling surface. Consider the production angle as well; a highly mass-produced mecha wouldn’t necessarily have many separate armour segments and these segments would be whole pieces, like a car’s hood. Cars’ hoods and frames are designed to deform on impact, and rounder shapes are safer for this rather than straight surfaces with angles. There’s something on certain lines and shapes that are naturally “safer” to our sensibilities, be it from experience or otherwise, and often using decent logic and “what looks good” attitude with an intended function can yield a design that works well.
While you can completely ignore the realities of manufacturing and process of it, it would add a touch of realism if there is some consideration whether or not certain shapes and angles are probable within the fiction.