I have heard that you gain a half inch in length for every 30 pounds you lose. I think it has an even more marked effect than that. Also, a gut hides your erection to a certain extent, whereas a flat stomach and the angle that it makes with your erection looks much more flattering from a size perspective.
If you look at Ron Jeremy’s old pornos as compared to his newer ones you can see the difference weight makes. Now he looks like he has a farely long, thick dick—before, when he was skinny, that thing looked massive.
A few years ago I lost about 30 pounds on a diet (I’m was already in decent shape). It was incredible how much longer my unit looked. Just for the record I have heard many girls say, “Skinny guys have big dicks”.