It was because in the beginning they wanted to put the idea that belters were different in people's head, but to do it the entire series would be too expensive.

It was because in the beginning they wanted to put the idea that belters were different in people's head, but to do it the entire series would be too expensive. They got rid of the issue in-universe with the bone-growth meds (?) that Miller and probably others took (in the books this didn't exist, and all belters are tall).