My DD(4.9yrs) is average weight and stature, but she has little breast buds too. She has a very obvious feminine shape too. I have a feeling she might mature early. I don't necessarily think that height or weight now would have alot to do with if they will mature earlier, it's more to do with their own hormones and probably heredity.
I went through puberty pretty early, I got my period at 11yrs old. I was also tall for my age then and had little breasts, which needed a bra around 11ish too. I was one of the first of my peers and I was always taller and bigger than them, from about 8- 13 (ish)yrs old. I then really slowed down growing (thank goodness!) and everything became average when I was about 13.