Market size is only a factor in specific cases and even then, it's not just the market, but the ability to have distribution within those markets.
Schools in Washington DC and New York City were added to the Big 10, not because of the schools but because of the added potential value to the distribution to the Big 10 Network. This only worked because that network was actually able to get wide distribution in those areas. The California schools, also in huge markets, didn't have that same value to the PAC 12 because their network has no distribution, people in Southern California, the Bay Area, Phoenix, Seattle, etc. can't see the damn network, yes some can, but enough can't that it was a big problem.
Texas sits in the 4th biggest media market in the state of Texas.
Ohio State is in the 3rd largest market in Ohio
Alabama is in the 5th largest market in Alabama.
etc etc etc