Well you can't buy people's votes. Not legally anyway. Also, the thing to keep in mind is that rally size isn't really meaningful. Trump was really impressed with his rally sizes because he hadn't seen anything like that before. He's not a public speaker so he doesn't appear before crowds like that iand it really impressed him. But his numbers weren't out of whack with what other candidates have drawn. Furthermore, crowd size doesn't really relate to vote share. Even a huge crowd of several thousand people is just a fraction of a fraction of the votes that a presidential candidate will get. Take Donald Trump, the largest crowd he pulled in was apparently about 30,000 people. That's a lot of people, far more than the couple hundred to couple thousand people he normally got. But there were over 120 million votes cast in the 2016 election. 30,000 people is about three one hundredths of one percent of that total. It's just irrelevant.