Why would a bodybuilder win against a boxer, that's a ridiculous question. You might as well ask who'd win a bodybuilding contest, a bodybuilder or a boxer!
They both train so differently, I train like a bodybuilder most of the time, cuz I am one, but i have trained with boxers and damn, it is tough! it's almost impossible to marry the two types of training.
If bodybuilders would win boxing matches then they'd be in the wrong business, a lot more money being a succesful boxer than BBer. Haha.
But to your presumed point of size versus training: Here, it's not boxing, but here's 150lbs BJJ guy versus 250lbs "Mr. Utah" in a mostly no-rules fight. Not to spoil it, but Mr. Utah loses [submits when caught in joint lock]. Combat sports are far more technical than people think, 100lbs size advantage makes a big difference, but doesn't make up for not 'knowing' what you're doing.
(This was back in the day when MMA was just materializing in the US. The BJJ guy isn't an mma fighter as far as I know, but Mr. Utah called the guy/school up and called him/them out apparently. Fun.) Sorry about the music.
The boxer would win without question. Unless the BBer manages to tackle the boxer and grapples with him on the ground maybe but even then.
As someone who used to box back in high school. I learned quickly it's not about size or strength at all. It's about skill and training. A line I use a lot; jokingly. "Never pick a fight with someone who gets hit in the face for fun".