As I have said, it you look here
https://en.m.wikipedia.org/wiki/Greater_Houston
You can see that their are 5 counties over 250,000 population, these counties make up 6,300,000 in these Counties nearly 80% of the population is connected to Houston. Galveston and Conroe are about the only large towns that are some distance to Houston not including Brazoria and even then I think Galveston is in Houston's UA and maybe Conroe. In Brazoria County their is an area of roughly 100,000 people or more of small towns that aren't connected at all to Houston but besides that Houston has very few large towns in its MSA that aren't part of the Urban Area. Dallas has several places, and the NE their are a plethora of towns and even small cities in an MSA that aren't physically connected to the city, this is not the case in Houston.