It's hard for us to tell in pictures how big she is. I know when I post pictures that sometimes I am amazed at how small the big cat looks or how big the small cat looks! She might just be a small kitty. My Stubby never gets over 6 to 7 lbs. If they are healthy and happy then I guess it's just their genetics. I just know that she has grown alot from her first few pics! She was such a tiny little victim.
You need to repost her first pics with pics of her now. That might be fun to see how she has changed.
How much does she weigh?