Well, I'm gonna go with the flow, pretend that it was suppose to happen.
When Ethan lived here, in the real world, he ate real food. Nobody who eats Dharma food is fat so it obviously can't be that bad for you so I'm going with, he lost weight on the island.
My official stance on this

is that the actor/Ethan (dunno his name) purposely gained and lost weight just to show the difference in him on island and off island.
That's my take!