Probably genetics it’s the most important thing here, but you could probably not discard ambient effects, in this case, the surrogate mother “ambient”. I don’t really know if this have been extensively studied, but a surrogate mother may contribute with hormones, factors or other active molecules that may at the end affect the size of the offspring. In genetics there is a law, that says that the phenotype is the sum of the genotype + the ambient. Probably this is one of such cases.