My apologies for blowing up your pixels and blurring them. Just did a quick resize in Photoshop so I could draw on them.

Arm: Currently the silhouette says to me extremely oversized deltoid (shoulder) muscles in comparison to anything else, seems to be no elbow joint and biceps and triceps are just stretched along either side of the arm. Without the elbow or notion of a joint, it looks like one continuous bone would run through the arm, just straight into the gun and I guess the gun has been built onto the end of the arm at an angle. I have outlined in red what seems more appropriate, perhaps the gun could be moved more along the forearm to accomodate for joint use. Elbow is fairly essential for the finished version to have realism.
Body: Did a quick sketch over the body of what I think I see right now and just an arm to better see the placement. The behemoth has no neck, but that's not so bad as with traps that big they would take over the neck anyway, especially when the head is bobbed down. Chest could be pushed out a little more. Back/lats aren't that big of an issue at a side on view theres not much you need to do for those. The abdominal muscles are squished as hell, but when we mutate like this they matter very little anyway. You did a good silhouette of the legs but if you want it to look really behemoth and mutated, you could shrink those to make the upper body look MASSIVE. Cause proportionately, the legs balance it out too much and if he stood up straight, he'd just looked like a ripped as giant rather than mutated and you know, like WIERDLY muscular. Like he developed muscles that don't normally exist with all these steroids and special treatments and scientific tests.
I hope this helps, I tried to share a bit of my knowledge but again I understand it's very hard to just pull this shit out of nowhere and make these improvements without doing the years of research/sketching/etc on muscles/anatomy. And even then, I don't know it that well. You did make good effort and improvement of the behemoth shape between your last post and the post before it though, so well done