As I mentioned in a thread yesterday, I have a Brother/Sister Carpet pair. The female was 1/2 the size of the male from 2 years old until 4. They both had identical food offerings, but the female would only eat 1/2 of what the male did. Then 2 years later the female just couldn’t get enough to eat this past year. Now she is the same length as her brother but without his girth.
As Deborah said, food intake and genetics are key.