@Johnny Bones is totally correct and provides the correct in-universe answer.
I just wanted to add a rather boring out of universe answer to say that the obvious reason is that Jorge Garcia, the actor who played Hurley, simply didn't lose much weight.
So whilst we can search for plenty of realistic in-universe reasons to justify this, the fact is he's a big guy with a serious obesity problem.
There are plenty of claims online that he was asked not to lose weight by the producers, as his character was so popular "as is" (and it gave joke-content on screen e.g. Sawyer mocking Hurley) - but I can't find any evidence this was the case.
Note: He does lose some weight after Season 1, just not much:
"I can afford to not live on rum and burritos and take better care of
myself...Besides, there were enough people who
were waiting for my character to lose weight as a result of being
stranded on a desert island."