Getting fat, per se, WILL make your dick look, smaller, I am an ex fat guy, trust me!
In my case, and many other guys I know who are big into powerlifting and for some in bodybuilding, do, infact, see an increase in penis size while training heavy. When lifting heavy, especially legs, it seems that something occurs that adds some size to the peins also.
The only logical answer I can come up with is that, during intensive muscular training testerone increases, therefore increasing the intensity and ability to achieve an erection. I have no proof to back this up, but I have had this conversation numerous times. So take it for what it is, another internet claim.