Help!!

I have 2 kittens, both sisters and from the same litter. ~10 months

Cat1 (calico) Is huge, was literally the runt of the litter and she is BIG Cat2 (orange cat) Is very tiny like I mean tiny, I need to know what I can do to help her gain weight and grow because we’ve been feeding her the same amount as her sister she just isn’t putting on weight or growth, it’s like she stopped growing at 7months I’m worried something is wrong but I don’t have the funds to get her a full work up and everything at the vet. Any advice? (Picture above)