Ignoring puberty that seems like a fairly decent answer. I'm a fair bit to the right on the curve as things go and 50'ish pounds seems a little closer for me but definitely not 75. Assuming ~175 detrained @ ~16-20% ish bodyfat and best natural training is about 220/225 at a similarly pudgy level.
If I was willing to have a stroke and eat my way to 300 pounds I'm certain I could put on more muscle but I'm fairly certain I wouldn't be able to hold onto to that extra muscle if I dieted back down again to 16-20%.
I think the Casey Butt study is a good representation of genetically gifted athletes on mild doses of steroids.
Your Maximum Muscular Bodyweight and Measurements - To think that any but a very few of us could outperform them is extremely naive/optimistic
A little
calculator for fun and discussion.