The number seems to be based on the fact that he made money of it. And if that was in the 100s of million, the fine should obviously be higher to ward of other people doing so (and not just have it as a cost of doing business). Kind of like the german movie piracy thing where the convicts had to give up thousands of bitcoin, which the state sold for more than 2 billion.
(Beside the fact that in other liberal democracies, he would be in prison now)
There were 2 issues. The first is that he made money off of it. The 2nd (and likely bigger issue) was that he repeatedly violated court orders (e.g. not complying with discovery, repeatedly lying under oath, threatening the jury on his show while the trial was going on, etc). Judges and juries generally really don't like it when one of the parties is lying their ass off and ignoring the judge's orders.
(Beside the fact that in other liberal democracies, he would be in prison now)