There's no doubting the health benefits of the traditional Mediterranean diet. I mean, it's not rocket science that a wide variety of fresh fruit and veg, a bit of dairy, fish and poultry, minimal red meat is better for you than processed food.
You're right that it's not just diet though...
I guess you're more likely to be eating and drinking at the same time in Spain, which certainly helps. I do still manage to get hangovers here thanks to my efforts.
Also, a mild hangover when you're on holiday with no work to do is always better than one where you've got to drag yourself out...