the problem is that there was no size tramophication problems in the cartoons
why did no one ever wonder wehre the trailer unit went when op chaged into his robot form?
why is it that soundwave was small enough to be picked up by spike in the pilot and yet he was big enough to rock on when he was normal?

these things don't matter so why should they matter in the film?

and prime needs to be a falt fronted truck, he's the ultimate hero the likes of which we'll never see so why change him?