Board Thread:Ask the Staff/@comment-27065637-20151107203749/@comment-26202330-20151107211739

LiviTheTepig wrote: It's because of how stable the fusion is. If the two have a good relationship and their personalities are compatable, they are stable. If not, then they will have extra body parts. Aye, that's my theory as well - the more compatible the Gems are, the more human the fusion appears. Conversely, the less compatible they are the more monstrous the fusion appears.

Just look at the least compatible fusion in the show - Malachite. She does not even have a basic humanoid shape (with arms and legs) but rather just has arms!