Rudenko is a smaller men, but being smaller is not only a disadvantage. Disadvantages of advantages and U-shaped curves are relevant here as well. Andryi is faster, more mobile, less of a target, and capable of changing head slots in a way bigger men cannot. Add to that a granite chin, and you get a tough mix to crack.
Also, Hrgovic was ducking under that hook. Zhang hit him in the back of the head and pulled further. Joyce was easy to find, but ultimately not dropped in the first fight. It was only the pitch-perfect shot in the rematch that dropped him -- a shot, that is not easily replicated on anybody not name Joe Joyce.
Rudenko went 12 with near-prime Povetkin and Lucas Browne and was never dropped. He has sturdy chin and nice high guard. In his two latest fights (against Sirenko and Anderson) he was clearly past his prime and wasn't badly hurt at any point. But he took too much punishment until the referees decided it's enough
Tony Tucker was 34-0 when he fought Tyson and also went the distance with Lewis whilst being aged 35. Tony Tucker was a top contender during his hey day and wasn't stopped until after 56 fights whilst being way past his prime, Tucker is known to be one of the more durable Heavyweights of his time. Certainly a level above Rudenko lets be honest, although Rudenko had never been stopped when he fought Zhang. And went the distance with some notable Heavyweights like Povetkin, so i don't really see that as a negative for Zhang in regards to his power.
Rudenko was an iron-chinned experienced gatekeeper while Zhang doesn't have quick feet or a good engine. Tyson didn't KO Tillis, Green, Ribalta (fake stoppage), Smith, Tucker, Douglas, Ruddock 2. Wilder didn't KO Stiverne 1. There's nothing unusual about this kind of thing.
Rudenko is an extremely durable, cagey journeyman. It’s very hard to KO these type of fighters. Anderson couldn’t even seriously hurt let alone put down an older, untrained Rudenko while throwing 100 punches a round.
A difference in strength & conditioning has affected Zhang's power. Since he Forest fight he got into heavy weightlifting with his s&c coach and you can see the difference in muscle mass. He then got a new s&c coach for Joyce rematch who dropped heavy weight lifting in favor of explosiveness training and using lighter weights for more conditioning. Also lots more cardio. Zhang's lack of cardio makes him seem like a lighter puncher than he is. With lack of cardio he doesn't commit much to his punches while throwing low volume at the same time making him a poor finisher. In the Joyce rematch we saw him throwing committed punches with mean intentions, and joyce was shook. Imagine if Zhang had the cardio to windmill like Wilder for 12 rounds
Lewis didn't KO old cruiserweight Ossie Ocasio, small journeyman Levi Billups, skinniest HW of all time Zjelko Mavrovic, couldn't put a dent on old Holyfield. One of the hardest punchers ever Earnie Shavers couldn't stop LHW Rondon. Tua couldn't stop Jeff Wooden etc.
I looked up Shavers' record the other day to see who he didn't stop. Stan Johnson (Shavers lost), Ron Stander (Shavers lost), Vicente Rondon, Jerry Quarry (Shavers lost), Bob Stallings (Shavers lost), Jimmy Young 2 (Shavers drew), Ron Lyle (Shavers lost), Henry Clark 1, Muhammad Ali (Shavers lost), Larry Holmes 1 (Shavers lost), Larry Holmes 2 (Shavers lost), Bernado Mercado (Shavers lost), Leroy Boone, Randall Cobb (Shavers lost), Ali Haakim, James Tillis (Shavers lost), Walter Santemore (Shavers lost), Rahim Muhammad, George Chaplin (Shavers lost). Brian Morgan and Brian Yates (Shavers lost) were ill-advised comeback fights. Shavers had a 77% KO ratio (despite facing a huge number of journeymen in his 91 fight career) and a 55% KO ratio against 215+ lbs opponents, the latter figure being very low by modern standards. Lewis didn't stop: 4 loss streak cruiser Ocasio, small journeyman Billups, crackhead Tucker, 24-5 crackhead McCall, 42 year old Holmes victim Mercer, fleet-footed 214 lbs journeyman-basher Mavrovic, former 190 pounder near-Ruiz Holyfield x2, 20 lbs overweight inactive 5’9 Tua, 8 years boxing experience Rahman
Yep I remember the fight with Lucas Browne who is a huge puncher. Rudenko has a solid chin and is very durable.