I would suggest that the big change is the amount of activity that an average kid does. which seems to be conducive to the calories in/vs out. the cause of this is complex. But a lot of it relates to stranger danger. Parents don't let their kids out after school unattended (for various reasons) which means they are inside not burning off as many calories.
The other thing to remember is that food is now cheap. Its perfectly possible to be in the bottom 20% of the UK income bracket and still have enough cash to eat more calories than you need. (This is a good thing. People starving in the UK is unforgivable.)