To use an admittedly rubbish analogy, going on holiday and packing on weight like that is a bit like running up debts. Pre season training is like saving money. Pretending that one doesn't impinge on the other is daft. During pre season, when he's supposed to be building up a bank of core fitness, which will stand to him over the course of a long season, he'll instead be spending the time - the early weeks at least - paying off the debts of the summer, instead of using that time, as most other players will, building reserves for a long season ahead. And when, as it invariably does with his fitness, it goes wrong, he has almost nothing to fall back on.
People talk about his 'natural' physique as if it's something that happens to him afresh every season. It's no different to guys who need to do loads of stretching for back issues or whatever. If he knows that he puts on weight easily and doesn't have the common sense to make sacrifices and adjust his eating habits and/or training regime when he's on holidays, then how can people expect him to do the right things at other times?