Most cities considered big cities have populations under 3 million. Philadelphia has 1.5 million people and have and support all 4 major sports plus MSL, the Arena Football League, indoor soccer when that was around, pro lacrosse, World Team Tennis, the AHL until a few years ago(the Phantoms now play in Lehigh Valley), several big named colleges, etc. Thing is, it's not just the city proper. Philly draws from South Jersey as well as Delaware. Hell, East Rutherford has like 10,000 people and has 2 NFL teams and until a couple of years ago had an NHL team(since moved to Newark) and an NBA team(since moved to Brooklyn).
It's better to look at the size of the MSA rather than the population of the city proper. From a ranking standpoint Vegas is the 29th largest with an estimated population of 2.1 million. That puts it ahead of Kansas City, Jacksonville, Cleveland, New Orleans, Milwaukee (still true even if you add the Green Bay MSA) and Indianapolis.
BTW Philadelphia has 6 million plus in their MSA while Phoenix has around 4.5 million.