In animals that develop without placentas (a chick inside an egg, for example), embryos have a fixed amount of nutrients. In mammals, however, an embryo gets nutrients from its mother. So it's in mom's interest to have a small baby, in order to spare her health. But dad's genes are greedy: All they care about is having big healthy babies. Mom's health is incidental, and so an evolutionary conflict arose. The ultimate resolution: babies that are middleweights. Hence, when Constancia deleted the paternal gene, the mother's silent gene won out and she got a smaller baby.