if you guys knew how politics works ,Trumps crowds would not impress you so much. The organizers have to find a hall that the candidate can fill. If the response is not good close to time, you move to a smaller arena. Then you send free tickets to everyone who is likely to go, to make sure. In Trump's case, they also pay people to show up. He has bus loads carrying people to the rally.
Trump always talks about the size of crowds. Why? Because the Trumpys will put as much significance in it as he does. The difference is he knows better. It is political gamesmanship.
I wait for one of his rallies and he starts calling the news people and cameramen terrible names, and they pack up and leave. He would stop endangering them.