Well yeah but it's a consistent fact throughout the series that fusions are always at least as tall/big as the combined sizes of the components (and sometimes much bigger like in Sugilite's or Malachite's cases). So if one component has grown, so should the fusion. Fusions embody the relationship between the components, and thus their appearance comes from how the components perceive each other. That's why Steg was totally muscular even though Greg is plump and Steven is stocky, because Steven and Greg both see each other as badasses (Steven sees his dad as an awesome rock-star, Greg sees Steven as an ultimate badass hero/savior and his son who he's immensely proud of).

Most likely, the reason why Stevonnie is so incredibly attractive is because Steven and Connie obviously see each other as attractive. And their physical perception of each other has grown and changed since both of them have gone through their adolescent growth-spurts; their physical perception of each other has upgraded, and so has their perception of their actual relationship in a physical sense. Therefore, Stevonnie should still be their combined size/height and thus should now be about eleven feet tall.

It's clearly just one of the many times when the Crewniverse didn't put thought into the sizes of character-designs. Rebecca and the Crewniverse are amazing, but they aren't perfect and do have flaws, and it's a well-known fact that one of their few flaws was always how off-model and inconsistent their portrayal of character-sizes tends to be.