My childhood pet, Lucky, was a runt and grew up to be a tiny little thing. She was always thin (though not to the point of being unhealthy) and small, but still a happy, robust girl. She was maybe, 6 pounds or so?
One of my current kitties is the same way; maybe 6 1/2 pounds, small but fluffy.
Personally, as long as she's got a clean bill of health and eats well, I wouldn't sweat it.
Proud parent of Scamper (age 12) and Robin (age 4)
At the rainbow bridge: Slippy, Faramir, Gabriel, Lucky, Ammy