It's really exccessive insest that causes that. If you have enough children, hopefully you will preserve most of the different genes you and your mate possess, but if you had say two children, then they had two children, and so on, you quickly reduce the varience of genes. What we learn from this: you better be turning out kids at a 1 every 9 months rate if you care about the survival of the species.

As for fat, there are no desk jobs when there are only two people in the world. Give it a year of hard labor hunting and gathering and building/working on a farm with you, and I'm sure the whole "she's too fat" problem goes away.