X3 I'm sure most people who watched Prime know about the slight unusualness of Arcee's regular form height and size... versus that of her motorcycle alt. mode.

C'mon Scrimscram, it's not that strange. Scream himself gets slightly larger when he poofs into a jet (because jets are... large. Very large. XD). I figure the logical explanation of this size-shifting would be that, being transformers they can either compact or expand their armor plating very efficiently in order to meet a certain size range for various alt. modes, and that certain body types are more able to take certain alt. mode types (smaller ones like Arcee can get as small as motorcycles or be as large as full-size regular cars, while much larger mechs can only go as small as normal car size and can go up and up to big military vehicles, planes, train engines, etc. >u<).

OR... you can just be startled like Scrimscram and go with the explanation of "Magics wut". X3c