I wouldn't be surprised. He seems to take this acting thing fairly seriously, and acting is all about being able to play off your fellow actors.
His job is made that much harder by having to work with someone who's never opened her mouth in front of a camera before. Looking mutely hot for ads is completely different.
But if it really was the weight thing, it REALLY makes no sense. The new girl is
smaller than Keira Knightley (look at her arms, and her thighs don't come near to touching), and you've already mentioned how skinny Keira is!

And at least Keira is able to gain some muscle tone. We have no idea if Rosie is able to do even that, there's no proof at all that she can put on more weight like Megan can.
I think it's just Bay throwing a fit and now throwing his weight around. He must be an idiot in all things non-visual moviemaking, if he doesn't know how stupid this makes him look.