They’re both pretty buff dudes even when off the movie set and relaxing. But lets analyse who would likely win in a fight and why.
I mean Damon’s ain’t no wilting flower, he’s a pretty strong guy. However for Wahlberg, I think fitness is more of a trademark/lifestyle sort of thing. Whereas Damon is a kinda big but not a bodybuilder level guy.
Height is in Damon’s favor, as he stands at about 5′10″ whereas Wahlberg is only about 5′7″ to 5′8″ — even in proportion to his frame, Wahlberg has relatively shorter arms. Damon has the height and reach advantage.
However Mark Wahlberg was in a gang as a kid, and he grew in a family of nine with shitload of brothers, so he likely learned how to throw a solid punch at an early age. He also learned how to take one. In contrast, Damon is the second son of two born to a stockbroker. Less of a hard knock life, and less likely to make a solid fighter out of the man, height and reach notwithstanding.
I’m giving the advantage to Wahlberg, I think he’d knock Damon out cold. Unless of course he went after that little dog Damon’s carrying… that would turn Good Will Hunting into Bad Will Hunting real damn fast.