If you put on somebody else's clothes do you become as big or little as they are? No, same case here, it's just stealing their clothes. Relax on the body size stuff, it will never change unless there was a way o make your character eat a bunch of food or work out.

So to figure out who of your friends are disguised enemies and who are trusted allies, you must keep your friends on a diet and consistently and routinely anolyse and memorise your friends' figures.

From my understanding, you appear to be of the body type you are disguised as, though you move as your natural body type; so a light disguised as a heavy, or the reverse, would be fairly obvious unless you compensate for the sake of appearances, not that a heavy could do much to appear to move like a light.

Or you could just look for the tard running away from battle and not fireing at the enemies. It'll either be a noob or an operative in disguise, either way it's good to kill it.

No, you stay your body type. It's not possible to have your body type adjusted in any way in this game (once it's picked). You put their clothes on, that's literally what it is.