The mom had gestational diabetes and didn't have prenatal care so that complicated things... I don't think that this is the first time that this happened but I hope it's not something that would happen to me...
Generally, moms who suffer from GDM tend to have bigger babies. Well in the mom's case, her GDM was uncontrolled thus the huge baby! Good thing both of them are just fine. But it's very irresponsible of a mom not to get any prenatal care whatsoever IMHO. Nuff said.
That is indeed very big! It's a good thing nothing wrong happened. Still... the mom should have had check ups etc. prior to giving birth. She had made a great risk doing that.