Flypen, why do you ask about the weight? There are two aspects, genotype and phenotype. Genotype is what the genes say, and there will be some variance. I don't know how many genes influence size in fowl, but quite a few, I would imagine. The phenotype is the outcome of those genes and their environment (feed, disease, weather), all of which can affect size. Weather is an interesting one, because when raising birds in a cold season, the chicks eat just to stay warm and grow. If too hot, food consumption drops, and so does growth. It is kind of like Goldilocks, if the temps are just right, the chicks eat well and grow well.
Feeding has a lot to do with the size of the miniatures I am playing with. Some really restrict the feed to keep them small. I am more interested in the genetic influence, so I weigh them and feed a standard diet.