This should be a rhetorical question...

[Holbytlass]

It almost seems that the two races' physique ought to be switched between hobbits and elves at least by their mobility. Meaning that hobbits seemed to do alot more physical work namely farming, digging out their holes and elves seem to be more sedentary with book/lore learning and even creating their homes with magic. Perhaps the elves actually did get out the ol' hammer and nails to build those flets and the hoes to raise the wheat(i'm assuming) for the lembas but it doesn't seem likely.

Then again there's fat and then there's fat. Hobbits may be rolypoly but overall I think them to be only 20-30 pounds over what todays standard says they ought to be, not too many of them being actually obese.

It's still perception, even in Tolkien's time. Skinny doesn't always mean healthy but healthy people who do eat right and are in tiptop physical condition are more toned and thinner. So it still makes sense for the elves to look thinner.