It's more than just good or bad foods leading to the increase.
Less kids exercising and playing sports (inside and outside of school), way way too much screen time (regardless of the medium), improper diet (referring to timing of meals and snacks), and beyond all that, and it's getting harder for dual-income households and the parents to put aside the time and extra money to be actively involved with the children's sports/exercise etc. It might not be that expensive if the child is just playing houseleague for fun, but if they happen to be good at the sports and play at the more elite levels, it can get pretty expensive and time-consuming.
Teachers used to run intramurals (lunchtime sports between classes) on their own time (and sometimes their own dime), which has pretty much become non-existent locally in schools nowadays, for which I have been told is primarily due to liability reasons.
Just because a kid is not fat, doesn't necessarily mean they are actually fit or in shape.