Size and strength have a lot to do with punching power, or else we wouldn't have weight classes. They're not the only factors, but if they didn't matter then weight classes wouldn't be a thing. It's why the heavyweight division was so popular for a long time: it produced a lot of knockouts. A lot of knock-out power comes from technique, but when you have two people with the same technique, the person capable of producing more torsion through their legs, hips and shoulders is going to generate more force. That'll be the larger/stronger person most of the time, which is why you don't see featherweights boxing against heavyweights.

This is also why people usually consider lighter weight classes to be a better show of skill: they have less knock out power, and are more reliant on technique to get the job done, whereas bigger fighters can throw their weight around to end a match. You can see this a lot in the UFC with its fighters. That's part of what made Tyson so exciting; he was exceptionally talented AND ridiculously explosive. He was a heavyweight with the foot/headwork of a boxer 20 pounds lighter than him.

The machines used to measure PSI from punches are also pretty spotty. You can throw the same punch and get wildly differing numbers, especially when you consider the difference in punching angles and styles. For instance, there are people who have popped up PSI numbers higher than Tyson's 1800~ who very clearly don't have the knock-out power that Tyson has. Tyson popped that number with a hook versus a superman punch or a big, lunging straight. If someone takes a great, big crow-hop into their punch, it's going to reflect differently than Tyson's tight, close-quarters hook.

Which is why measuring PSI is largely useless as a comparison unless everyone is throwing the exact same punch -- and that's probably not going to happen.