Heavyweight is a slight exception to the rule because there is no upper weight limit and you get a wider variety of athletes. Andy Ruiz is a very talented boxer and even when fat was mentally sharp and strong. His stamina would have taken a big hit obviously. He's also 34-2 with 22 knockout wins and one of the best heavyweights in the world. I wouldn't take what he did after the AJ win to be determinant of his overall prowess.
This bit makes me think that it's not boxing. The one was of winning is to hit your opponent really hard so they stop and that can happen any time. So, training-wise you're splitting your time between hitting well and general fitness, whereas in other sports, fitness is a much greater requirement. Plus, tennis has X number of sets and football has 90 minutes, but a boxing match could be done in 3 minutes.
I would say that the training styles of the various sports are probably the most indicative of the fitness levels of a sport. You've got sports like darts, purely about skill, sports like weightlifting or sprinting, mainly about power, sports focused on endurance, and hybrids
It really depends on what ratio you deem important.
I reckon we should work it out by saying "which sportsman would survive the longest in neolithic times?". That would be all round fit