The shoulders are bowing out like that because they're heavy and nothing is stopping the 'piston' from obeying them (and gravity). As for the arm vs. shoulder thing, I think he was asking because how you position yourself inside the suit can affect how the suit flexes and moves.
In this particular case, I would bet that the 'fix' involves bracing the piston-devices. Do you have any padding between them and your actual body? Step #1 is giving the piston a flat, rigid surface to act upon (think of it being like trying to stand up and reach for something while on a water bed)...without the rigid surface, there's really nothing to hold the piston upright. Once you solve that, Step #2....you need to make sure that this rigid surface is as close to level (that is, parallel with the ground you stand on) as you can get it). It's not going to help you having a vertical mount if the mount itself is still slanted at 45 degrees. Once you have the surface rigid, and the mount vertical, then (from the pic) I would say you need to either adjust where the mount attaches to the shoulder, or simply pad the inside of the shoulder so that there's more 'stuff' between it and the mount. That way, it's not nearly so easy for the shoulder itself to pull the entire suit down.
As for specifics...if you take a look at Sylgian's build, he has a few great photos of how he reinforced the 'skeleton' of his chestplate. This would help make everything less flexible (foam builds are squishy) and give the piston thing that flat, rigid surface to act on. As for inside the shoulder itself...experiment? Assuming you have some foam left over, just try building up the area around the point where the mount connects to the shoulder to lessen it's range of motion.