It's mostly this

Quote Originally Posted by Bidoofdude View Post
do it to make them look bigger
Off the top of my head I don't recall any character explicitly mentioning a particular size change in the fiction, except that they form "giant robots".

Scale is always a funny thing with Transformers. Groove is a motorcycle (maybe 7 feet long) and Blast Off is a space shuttle (184 feet long) and they both form similar size limbs and transform into similar size robots.