As a biology/neuro major this question caught my eye. The problem with the idea of things being genetic(especially traits like size) is that people assume genetic determinism. If a snake has big genes, it gets big. Two big snakes make two big snakes right? Size in animals comes from a variety of factors, both nature and nurture. If you fed one twin more often, and kept it in a slightly larger enclosure and compared that to another twin fed slightly less, with a smaller enclosure, less heat, I would assume that even at full growth you'll notice a size difference. The best way to think of it is that the genetics give it a predisposition to size. When you add nurture, you're adding a second variable that is highly intertwined with the first.

I hope this made sense!