If fatness is mainly genetic then how come the percentage of fat people has increased as society has "progressed" towards current year?
Yes. There is a genetic factor.
But the fact is that if only good healthy food was available, if gyms were free and mandatory, if schools were geared up to promote good health... I think that common sense dictates that this would lead to a massive environmental factor which would mean that even those who have a genetically lower metabolism would not be so fat.
In the end fatness is simple problem. It really is just calories in vs calories out. The genetic factor is that some people have a higher basal metabolism which means that they burn more calories even without exercise, and they absorb fewer calories from the food which they input, hence why they remain slim.
If we do things such as ban the fast food chains from providing killer foods, just like we ban them from selling heroine, then this obesity problem would not exist.