I liked Hurley's own comment about it, in an episode in season one. If I recall, he was talking to Charlie, who had questioned why Hurley's weight loss wasn't more evident. He said something to the effect of "Dude - I'm a big guy. It's going to take a while."
Basically, this is just one of those things you need to suspend belief on. Yes, he probably should be looking more thin. Yes, Walt shouldn't be looking dramatically older (take a look at some stills of him from early season 1 and compare). Yes, their hair should all be growing out. Etc. The writers do what they can, and occasionally show people shaving, getting haircuts, etc.
If there's truly going to be a February sweeps explanation for Hurley's weight I'm curious to see it, but I suspect Damon and Carlton were just having a little fun when they said there would be "definitive" explanation.