Dave Meltzer on quoting the sports business journal on boxing's fan base:
Those numbers have increased over the past decade from 37.4 million to 50.7 million, and will continue to increase over the next decade.
This chart below shows numerous other sports in the US relative to the UFC curtosy of MMA payout. They also include soccer which has around 21 million fans.
The size of the UFC fan base in the United States is estimated to be approximately 31 million people. You can see below how this compares to the size of the general fan base of the other major sports played in the US over the last three years.
So clearly as we can see boxing is more than two times the size of soccer. Boxing does on the other hand have the unfortunate problem of having a ****, lazy as **** ignorant fan base.