Look, tits aside, this man is DIRTY.
Like, black patches starting to build up on skin type dirty. Dude hasn't showered in several weeks.
I know Simon just got back from prison, but that aside, the standard level of grime on people must fucking suck. The COI folk were certainly cleaner than him. It must be pretty bad in a scarcity crisis like this.
But, aside from being freshly banged-up, I actually don't see any major scars (that line looks like an impact with the wall/hot pipe. We know the Iron Lung's walls were hot enough to boil condensation drops).
Since we know about how effective the Butcher was... yeah that checks out. I seriously doubt he's been overpowered often, considering literally everyone else is in some kind of calorie deficit (supposedly), and Simon is massive. Any time I try to explain that, it doesn't point to anything good:
Simon either stole enough food to stave off withering away in jail (which means he was playing dirty in there... I don't put that past him)
The COI was genuinely feeding him (I don't buy it)
He was eating good on Eden (equally as frightening to think about), and didn't spend much time in COI custody before getting put in the Iron Lung
Paraphrasing Markiplier, he wanted Simon to have a "rugged survivor build", hence getting into the best shape of his life for the role.
That also implies Simon was lowkey in the best shape of his life for this expedition (or possibly Filament's Battle). Dude was absolutely hogging the gym in prison. I laugh at the thought of Simon walking in there and everyone else getting up to leave immediately because they don't want to deal with his erratic bullshit LOL
EDIT: I've been corrected multiple times, so I'm putting this down for self-reference. He's probably jacked because the COI used him for labor/work. Directly implied by "how many times are you going to use me before you let me go?"
I assume, then, that Simon had been doing risky stuff for them for a hot minute, and the SM-13 was just more of the same. Of course it wouldn't have clicked in his head this was an execution at first. He probably "came back" from several various excursions in the past. Most likely, less extreme as this.