I'd imagine many things are to blame for kids (and adults) getting larger.
For a start the availability of fast food is far great than it was in my youth (70/80's). Also thinking back for my mates and I things like fizzy drinks and sweets were a once or twice a week treat. These days for many that is an everyday occurrence. It also ties in with the fact the UK has by far and away the highest level of chocolate and fizzy drink consumption in Europe.
The French comparison is interesting, but then like the example given when I was young people did eat smaller versions of their parents meals. I know I did. I can imagine I'd have been wearing my dinner if I suggested I should have been eating something else to my mum! The modern thinking of kids meals is nothing but marketing. Until some marketing folk convinced folk it was the done thing kids just eat smaller portions of their parents meal. Why wouldn't they? Do you think in India a young child is eating a small plate of curry that their parents eat, or some animal shaped chicken nuggets?
I also don't believe in the climate argument. I'd say parental fear is a bigger issue. When I was a kid everyone walked to school. Not one person at my school was driven to school. Now if you go by the same school at opening or closing time there is a convoy of Chelsea tractors waiting to ferry the kids about.
Add to that the modern thinking that everything should be some sort of official organised event. When I was a kid we just went out to play. Sometimes it was sports, other times it was just messing about in the street. These days more and more kids just seem to get shuttled between friends houses, not allowed out as it's to dangerous.
Technology will also play a role. I'll admit I clocked up many an hour playing games on the Speccy or C64 back in the day. Although if I wanted to see a mate or find out what they were doing I had to get up and out the house and go chap the door. These days they can just text, email or Facebook them. No need to even get up off the couch.
So you have a less active population, easier access to and higher consumption of high calorie foods and people wonder why folk are getting fatter! We feed kids more calories and there is a culture of fear that you are a bad parent if you let your kid outside to do something that isn't an organised event at a school or club. Yeah, the kids will get fat.