It's not always clear. Megafauna, despite being huge and seemingly almost unstoppable, are actually often more likely to die because they are less adaptable, and they require such a vast amount of food to sustain their size. Climate often kills them. A small change in climate can change food distribution a lot. Smaller animals can survive in smaller numbers, or can adapt better. Large animals just die. The other main thing is people. People are, without any doubt, the best killers the planet has ever seen. We are mind-numbingly good at killing other things! So when climate changes, and people arrive, big things and/or threats to people tend not to last long. Usually there is a combination of effects which add together and take out a species. If only the same thing happened to the hams. Personally I would totally support monthly ham-hunts where people armed with spears hunt down the hams to keep number in check. Only thing is that it might be a little easy. Hams can't run and can't fight. The ham-hunts would be very one sided, but probably still fun :)