Rocky was very strong for his size. Though athletic wise I think you'd be disappointed. I've read where quite a few of his boyhood friends could throw a baseball and football farther than he could.
No offense but I sincerely doubt Marciano hit harder than Louis .. any sparring partner that sparred w them both sparred w an old Louis .. take a look at Louis lifting a 240 Galento off his feet into the air with a hook or the shots that destroyed giants like Max Baer, Buddy Baer or Abe Simon .. Marciano loaded up with every shot but Louis along w Sam Langford was an all time pound for pound puncher .. http://www.historicfilms.com/search...ento&q=joe+louis+tony+galento#p1t10797l732261
That being said Marciano was a pretty good natural, all around athlete .. he played football, he played baseball .. he was obviously very strong in his legs and core .. off the chart well conditioned .. what more van you ask ?
From the video I've seen online Joshua does sets of 7, I doubt he could do anywhere near 30 in one set, probably not even 20. I do sets of 11x3 with a 20kg weight and can only do 26 pullups max. Bigger guys like Joshua will always struggle with body weight exercises they have so much more to lift and with longer limbs will have a bigger range of motion.
In terms of reps, of course it favours lighter weight guys. Marciano was quite a heavy man himself, so 30 reps is great. But a "super heavyweight" weighs 240 pounds or whatever, so they aren't going to be able to compete.
What don't you understand about smaller men having a better power to weigh ratio (which aids bodyweight exercises) And Marciano's short limbs also help with bodyweight exercises since there is a smaller range of motion. If you "don't buy that **** that it favors lighter weight guys" can you explain why gymnastics is compromised primarily of midgets and not 6'5 roid beasts like Joshua?
Joshua certainly has greater absolute strength than Marciano. High reps with bodyweight exercises are about power to weight ratio and range of motion. Being lighter and having shorter limbs is an advantage if all you care about is reps.