There was no 200 pound limit in his day, it was 175 then. I don't think he'd campaign at HW today. I do think there's a couple of men here and there he has a chance of defeating but that depends on whether or not he can carry the weight. Men of his frame have bulked up with success since his day but it doesn't guarantee he does.
I'm talking about when he fought Shkor. Marciano was outweighed by 30lb and had the opportunity to bulk up for that fight but still came in at his usual weight.
If you can get the job done without adding the weight that's the ideal scenario isn't it. I don't think today he could get the job done at 180 pounds.
Or maybe he knew that was his optimum fighting weight and bulking up would have been bad for his performance.
Every fighter has an optimum weight and they will trade off some skill for strength as they bulk up if they want to compete in a higher division.
This is so simple to answer. Why didn't Rocky bulk up when he gave away 30lb fighting Shkor? Because this was 33 years before the Micheal Spinks experiment. Those Methods had not been invented yet. It was not possible to gain functional artificial weight until that point.
Maybe he thought a guy who had been knocked spark out by Walcott in1.34min of the first round in his last fight ,and had won only won three of his last eleven fights wouldn't be too much of a threat ,and so wasn't worth all the hassle?
naturally they do. Traditionally there is such a thing as optimal weight, but today things have advanced to "optimal weight for each fight". A different programme will achieve another optimal weight to suit a game plan. Check out Orlin Norris. That guy flipped between 189lb and 240lb back and forth. Changed his body for each assignment. He fought SHW guys like Golota and Waldo after being a cruiser champ then went back to cruiser. Interestingly, the guy was the same size as Marciano. Maybe one inch longer arms and one inch shorter in height. At the end of the day the way 240 is achieved today, how functional that is today, was impossible before the Spinks experiment. These "SHW with skill" just wouldn't exist as we know them now because they didn't before. To be strong enough to fight under traditional methods they were oafs.
That's how it worked out, and it is just as well because it was not possible to bulk up artificially in any kind of way that would have been an asset in 1951.