If she's acting normal, there's nothing wrong. Was she the runt of the litter that you got her from? I have a cat that's over a year old now, and she's still small and hasn't really grown in the past couple months. We named her Bit due to her small size and squeaky meow. Bit was the runt of the litter that she came from, so I'm thinking that may have contributed to her small size. I wouldn't worry too much about it, but you might consider getting her a checkup anyway.
I hope this helps :)
Owned by a lovable runt