Walker wasn't actually KOed though, and kept attacking in the 8th round, it's not like he got blasted out, he showed a tremendous chin in that fight. I don't doubt Schmeling's power was a different level to McCellen's.
Bearcat Wright was 210 pounds of muscle and could punch. Tiger Flowers had McClellan's reach. King Levinksy could punch and box a bit, good enough to beat Tommy Loughran and Jimmy Slattery. Uzcudun was 195 and could punch. OK, there are some reasons based on ring performance that provide evidence to Walker's ability in this match. And who did McClellan beat again? Who that was as skilled as Walker? Who as accomplished as Walker?