Hurley was on the island for 3 months. While technically he should've lost a lot of weight from all the walking, sweating, smaller food portions, and no refined sugars he did have access to the Dharma food stores to sort of explain him maintaining his weight. Then he went back to LA for 3 years and probably ate a lot to keep his figure.
It's just one of those things, like hair growth, that you have to suspend disbelief on.
