Harry Greb's contemporaries generally had lifetime knockout percentages which were far from impressive by recent standards. Maxie Rosenbloom, Tommy Loughran and Dave Shade had far lower lifetime knockout percentages than Greb did. So Greb was not even the softest puncher at the time he was active, let alone in the history of boxing. I still wouldn't say that modern fighters were better punchers than fighters of Greb's era solely based on knockout percentages because there have been some changes in boxing over the last 90 or 100 years. One big difference is that referees have a tendency to stop bouts far more often and much sooner that they did during Greb's time. - Chuck Johnston
I really do not think that knockout percentage, is any sort of indicator of power. I think that his is much more closely correlated to a fighters level of competition, with fighters who had a higher percentage of fights against elite fighters, typically having lower knockout percentages. No while I don't think that Greb was a particularly hard puncher, his ridiculous level of competition, would be enough to explain his low knockout percentage.