Any overweight person. I gained 10% of my bodyweight in the last 6 months and I haven't felt better. (Mostly eating plants though.)
The complex people have around weight I find really strange.
I'd be surprised if most of the mental health effects don't simply follow from the realisation that you are in control of your own weight.
Feeling powerless is pretty shit.
If you're obese, changing composition will likely add many health benefits, and very often the overall weight of the person shifting said composition will be lower because they were carrying a lot of fat, and only added a reasonable amount of muscle if they took up exercise.
Also, the vast majority of body builders are in “overweight”. Most muscle heavy gym goers should still be in normal BMI, or maybe the low end of overweight at a 10-15% body fat.
Should optimally be <0.5.