This was done as part of an album cover for a music artist:
https://www.behance.net/gallery/52350045/Empire-on-the-Run-Fat-Boy
Fat Man was fat because he was a spherical implosion, plutonium weapon.
Little Boy was skinny because he was a ‘gun’ uranium design .
I’d suggest making the main texture more metallic and soften the bump/normal a bit.
A few more loop cuts in the area of the fins would help too.