Honestly, this is essentially BS. I'd say that 95% of healthy weight people stay that weight because their metabolism and appetite works properly. People who stay in shape by paying careful attention to their diet are a tiny minority.
Why would you ask old people if young people are healthier than they were? Surely, Spain compiles medical statistics.
Anyway, Spain appears to have the same weight issues as everywhere else:
Those numbers are relying on how long the older generation lives. If the younger people are physically more active, how do you know that they will also live as long? It could be that modern expectations of what a "healthy" body should look like takes it too far, and that you will end up with a shorter life.
I see a lot of former athletes dying in their 60s/70s from heart failures and other conditions. Perhaps the body has limits to how much abuse it can take.
Source: have to do keto diet and be strict to stop food addiction.