All the research I've done (basically Google searches) says that losing weight will really only make your penis LOOK bigger, but it won't actually grow in size.
The "fat pad" compresses easily during sex allowing most of your cock length to penetrate... depending on HOW overweight you are, of course.
For some, I suppose that's enough, but for our wives wanting deeper penetration, it really will only get you a few millimeters further in - maybe a quarter inch - DEFINITELY not enough to please MY wife who craves that "deep" orgasm that I've never given her, and WILL never give her - at least not by MY penis.

I know my wife likes the look of a long flaccid penis - she has commented on those she's seen with great excitement, much to my chagrin prior to discovering my long-latent cuckold desires.
Wannabe - see my captions at http://www.cuckoldplace.com/27_74912_1.html