I'm trying my damndest to think of a legit HW who has 5 rounds of strong cardio (by "legit", I always mean someone unable to make the nearest lower weight class, at least without being massively drained or comically weak. So for HWs, that means unable to make 205 without chopping limbs).
It's a short list---
Cain
JDS
Minotauro
I'm missing people for sure, and maybe the "legit" tag is too constraining for the HW's. Without that limitation, guys like Fedor can make the list, and the list gets pretty big from there.
But anyway. It's well-known that naturally big people have problems getting oxygen to all that mass (I guess artificially muscular people would have the same problem). Now, I'm not sure where EPO testing stands these days, if they're testing and catching people or what, but if a fighter can safely do EPO, then that would obviously help cardio a ton, regardless of how big. There are other substances too, but I'm sure they're tested for. GW 50516 and SR 9009 being two off the bat. 9009 is very short acting but potent in high doses (the literature says it only barely absorbs when ingested by pill or liquid, but it doesn't matter, because it flat out WORKS and gives massive short-term increase in cardio. Short term as in, I'd say up to a couple of hours after ingestion).
It seems even with PED's, combat endurance royalty runs at 170 or below. Whatever the case, suffice to say Greg Hardy ain't wearing a cardio kang crown.