The benefits of calorie restriction are only partially caused by carrying less fat (specifically visceral fat that triggers higher levels of chronic inflammation). Here is a demonstration of what happens when you remove visceral fat from mice, for example. You get life extension, but not as much as calorie restriction:

http://www.fightaging.org/archives/2008/03/lose-the-visceral...

Calorie restriction is doing other things to metabolism as well. For example, autophagy is upregulated by calorie restriction. Upregulating autophagy by other means enhances life span. See:

http://www.fightaging.org/archives/2009/10/attention-paid-to...

http://www.fightaging.org/archives/2007/10/calorie-restricti...

There are probably other line items as well, but given that calorie restriction doesn't work in animals with disabled autophagy mechanisms, it's the place to be looking. The plausible theory would be that increased autophagy cleans up damaged mitochondria, and mitochondrial function and damage are clearly very important determinants of longevity. See:

http://www.fightaging.org/archives/2006/10/how-age-damaged-m...

http://www.fightaging.org/archives/2008/01/revisiting-skulac...