If you know the history of megafauna mammals then the end of them is obvious.
"Subsequent to the Cretaceous - Paleogene extinction event that eliminated the non-avian dinosaurs about 66 Ma ago, terrestrial mammals underwent a nearly exponential increase in body size as they diversified to occupy the ecological niches left vacant. Starting from just a few kg before the event, maximum size had reached ~50 kg a few million years later, and ~750 kg by the end of the Paleocene. This trend of increasing body mass appears to level off about 40 Ma ago (in the late Eocene), suggesting that physiological or ecological constraints had been reached, after an increase in body mass of over three orders of magnitude. However, when considered from the standpoint of rate of size increase per generation, the exponential increase is found to have continued until the appearance of Indricotherium 30 Ma ago."
So, they got bigger because there was a sudden niche to exploit. The niche ended and eventually they died off as the advantage was lost. Did humans effect the timescale of this, maybe. Did humans change the course of nature, almost certainly not, as if humans aren't part of nature.
BTW, when did the avian dinosaurs go extinct? I can't find that fact.