No contest.
The 125 pro boxer guy probably hits harder than an athletic 200lb dude. Most average big guys can't box for shlt, they telegraph their punches trying to leverage that weight which makes them slower and weaker. A pro 125lb boxer who is used to fighting other 125lb wasp like pros would see those punches coming long before the punch arrives and would step inside and eat them for breakfast.
I have some minor boxing training from my teens. Trained for 3 years and I can dance around my friends when joking around even though I'm not sharp anymore. If I'm fighting a big guy in the street, I'm not even going to take a chance on my power dropping him on the chin. I'm aiming for a one shot to the liver while he inevitably overextends on his telegraphed weak haymaker. I'm nowhere near pro level but would fancy my chances against any untrained fighter bar a lucky punch.
Your average guy
Can't punch with power
Isn't accurate
Telegraphs everything
Gases quickly
Leaves themselves wide open
Can't take a body shot no matter how big they are