My DD is 7 months old and very small for her age (she still fits into clothes meant for 0-3 months) and was tracking along the 9th centile at her last 2 visits to baby clinic. She is bright and smiley, is babbling (ga,ga, ba,ba, etc) can hold her head up and roll in both directions but she can't sit up unaided (and wobbles if you hold her hands) and is definitely nowhere near crawling (although she can work her way round to pick up a toy that's near by if she wants it).
I guess I just worry that her size might be symptomatic of some other kind of problem especially as she wasn't small at birth (50th centile). She eats well, milk and three meals of various solids per day (mostly pureed stuff). Is her progress normal? (our doctor doesn't seem worried as she seems healthy and looks chunky, just small).