I used to think it was purely ego, but recently another thought has occurred.

He has to attack the crowd sizes because it shows how many Americans are out there to vote for her. In the places that most of his voters live, they’re surrounded by other Trumpets, and they put up lots of signs and are super loud about it. Which creates the illusion that nobody is voting for the Dems. This makes the claims that the election is rigged or stolen more credible to his followers. I hear it from my local trumpets all the time; “Nobody I know voted for Biden”, implying it had to be stolen.

So if there is all this coverage of how big her crowds are, it creates more cognitive dissonance for (at least some of) his followers.

But it also could just be his narcissism and ego. I don’t really know.