What if they're slim most of the time using their radiant powers that allow them to manipulate their body type since its more convenient for gun handling and movement but for certain abilities they can revert to their original form like mr fantastic's ultimate ^_^
I'm all for representation but i believe from a tactical perspective it makes no sense: the person playing this agent will always be at a disadvantage. Its the same problem as when a plus sized person tries to sit in an aeroplane; it simply won't work.