weight gain can make your penis LOOK smaller because your fat pad over your pubic bone gets thicker. it sounds like this isn't the case for our goggleguy friend. is it?
mine shrank and it was not the case. i don't know exactly what caused it, could have been poor diet, depression, becoming less fit (but not fat), or who knows. my drive was definitely down though and i could feel it. and one day, i just looked down at it and my defeated libido was physically showing (my dick got smaller).
my advice is go to the gym, try to improve your diet some, jelq, and generally just try to get stoked about life. maybe try to start dating more if you're not seeing adequate action ... you know, to get the blood circulating everywhere it has to.
good luck man.