User blog comment:PlantyThePottedPlant/Random Topic Blogging II: Electric Blogaloo/@comment-76.164.106.89-20120725195538/@comment-2077777-20120726152826

Because it's a melon. It's round and only has one shape, so it was quicker for it to grow. Candace on the other hand has a nose, ear, 10 toes, two arms, and many organs that also need to grow. So each body part was taking it's time to ajust to the growth serum.