This is an incredibly misleading statistic. When you describe "a town of less than 50,000 people," what comes to mind most easily is some sort of rural town. But at your link, I recognize several of the places on that "fastest-growing" list. Leander is a suburb of Austin, Little Elm and Forney are both suburbs of Dallas, as are (scrolling down) Frisco and Farmers Branch, while Boerne is a suburb of San Antonio, and Georgetown is a suburb of Austin again.
Most of these, you wouldn't know you were in a separate city if you didn't pay close attention to roadside signs, and I suspect the same is true of a large percentage of that 61% figure.
That says something, since those people didn't choose to live closer to the central of the metro area, but it doesn't say as much as you're implying it does, because they're all examples of places for which, if someone outside of Texas asks where they live, they're going to say "Austin" or "Dallas" or "San Antonio," and only if asked where in Dallas will they respond Little Elm, just as they might otherwise respond Lakewood or Hamilton Park, which are neighborhoods but not separate cities.