If they are out of the same litter, its just their personalities - she's a more dominate kitten.
Was she the only female in the litter? Maybe she is thinking more like a male
I know Charlie was the dominate one in the litter - breeder told me so. And he will be a bigger male Oci - 11-12 lbs (he's about 10 lbs now). Oci males will go 9-12 (sometimes 13 or 14 but thats not the norm). The females can range from 8-10 lbs, so he might wind up being 9 lbs and she might be 10 lbs.
How old are they now and how much does each weigh? Charlie weighed 5 lbs at 4 months old.