Thanks to some research by Richard Ryley over at Fade's site, it's finally been determined what effect Form has on your monster. I can't put up the details yet because it haven't asked RR's permission but to sum it up:

Skinny monster = Higher evade rate, but takes more damage

Fat monster = Lower evade rate, but take less damage

The reason nobody found any effect is bexause we were looking in the woring places! The effects of Form show in the Battle Arena, not on the Ranch :)

but being skinny and fat also reduces lifespan right? or has that school of thought changed?

So far the tests have indicated no loss of lifespan, as far as we can tell.