Rivet's simple cargo plane form looks to be becoming a BOeING...okay so what is happening here is, under her metal in this form is what I like to think of as a rubbery cargo bay. Her whole body under it is mere rubber so it allows her to inflate. Of course it is bad if you're lugging cargo that when mixed (cola and mentos) will cause said cargo bay to balloon and pop your rivets like they're buttons XD