how old is the lizard? it looks awfully skinny, and if it eats like u say it does and has not gained any weight there is a possibility it could have internal parisites, and may just be very resillient to them. i would have a fecal exam done by a Vet, to see if he has everything in order in that manner, as for the bulge, my initial response is that it appears to be the developing of the hemipenes, but it does look a little oddly shaped, and as if it is only on one side, clearer pictures are needed of the underbelly from straghit on. A good way to get that shot is by putting it against some glass or see through plastic, so that we can get a clearer viewpoint. also if Randy from NeoScales posts on here im sure he will post a link about internal parisites and crested geckos that will be helpful
Andrew Ellis