Each baby is different. Your kitten could just have it in her genes that she be a small kitty.

My Samantha is very small to me, she is the smallest I have at 8#, Chester is is only several months older weighs in at a whopping 17#. Her bone structure is smaller, she is not as tall or as long..... but she isn't skinney looking. BUT she is the most playful of my 4 and she is not napping during the day while the rest are. (she is to busy watching for me to sit down so she can get in my lap)