True, soccer is probably one of the few top level, lucrative sports a smaller guy can get into, but like you said, it's very competitive. That, and the amount of "soccer sized" people are a dime a dozen, so there's no shortage of athletes. So, from your point if MMA/Boxing is getting 2nd tier athletes at the smaller weights, then HW is probably getting 4th tier or worse (on average).
Baseball players tend to be bigger than you might think, particularly pitchers.
That's almost certainly true for North America, I wonder how true it is for the rest of the world (seriously, I don't know). Other than soccer, what sports pay big outside North America?
Tennis, but being tall seems to be an advantage there. Cricket I'd guess in India, but if size is important in baseball, I'd guess its important in cricket too. Size is important in rugby, and Australia has its own version of football to boot. And size is important in hockey (northern Europe and Russia) But how many positions are there for those sports?
Come to think of it, your average WW walks around at a fit 190-200 pounds. Meaning most WW's would be an acceptable size for a skilled hockey or baseball player - and since even the guys sitting on the bench in the NHL or MLB make a million+ a year, perhaps even WW and up get the 2nd choice in terms of athletic ability. The chances are that for instance your average NHL guy weighing 190 pounds is more athletic than GSP (the most athletic kids in Canada play hockey - trust me on this, its not even close).
Meaning HW might not be as unique in not having top talent as people suggest - anyone from WW up who is athletically exceptional is probably playing one of the top team sports (soccer in most of the world, hockey or baseball in NA).
Which might be an argument for watching the lower weight divisions (LW and under); they're probably the only ones that draw from the most athletic folks in the world in their size range.
Of course the whole question of how much general athleticism crosses over into fighting comes up at that point. The ability (and willingness) to take a punch is pretty selective, both physically and mentally. Crosbie is almost certainly much more athletic than GSP is (and is about the same size), but even if he weren't making much more than GSP playing hockey I'm not sure his athleticism would translate into MMA. Same for someone like say Lebron James vs Cain.