Though it's impossible to ascertain with complete accuracy how UK's basketball fanbase compares numerically with some of the big football powerhouses (Texas, Florida, Ohio State, etc.) mentioned, there are some elements in play here that might make it much closer than many would think, based merely on the populations of those states.
Someone mentioned "those that care about college sports," and I think you'd find that percentage is higher- in some cases much higher- here than in places like Texas, Florida, etc. Between pro sports, transplanted populations (particularly in the sun belt), and competing schools, the percentage of those who might claim allegiance to those programs is likely smaller than those who profess to be UK fans here.
The CJ conducted a "Bluegrass State Poll" nearly 30 years ago, and I remember in the article about it that it was considered astonishing that fully 1 in 2 adult Kentuckians polled claimed to be avid Wildcat fans. That may not seem remarkable, but in truth it would be difficult to go anywhere in the US and discover that more than 50%- or anything close- of those polled would claim to be avid fans of any one team, college or pro. It's hard to believe for fanatics like us, but there are just a lot of people everywhere who don't care about sports very much at all.
Also, as our detractors love to point out, aside from not having pro sports, there really isn't as much to do here as in some of these populous, more cosmopolitan states. Again, that's relative; it's not like there's nothing to do here, but compared to, say, Florida or Texas- warm weather states with multiple huge cities- there isn't. I don't personally consider that a negative- it just makes us more fanatical about our beloved Cats, and it makes visiting those places more exciting.
In short, the extreme passion for Kentucky Basketball that has permeated this state and the diaspora of its sons and daughters through so many decades may not quite make up for the sheer numbers in some of these much larger places, but I'd be willing to bet anything it closes the gap more than many might believe.