[KevinFromTexas]

Strictly going by the facts and the way that metro areas are determined officially by the government (US Census Bureau), San Marcos is part of Austin's metro, while New Braunfels is part of San Antonio's metro. They determine which cities and towns are part of which metros based on commute patterns. If more people are commuting to "City A" than to "City B", then it's part of City A's metro.

That seems like it can get tricky, though, and there can be economic issues that affect this. For instance, let's say one city is more economically healthy than another, or just a bigger city with more employers. In some cases you might even have a small town that is closer to say City A than to City B, but the bulk of that town's residents work in City B, and so it's part of City B's metro even though it's closer to City A.

This is why when cities get even bigger with more economic power and jobs they end up with a huge amount of counties in their metro. Just look at the Northeast, California and Florida. DC and Baltimore are a good example. DC is a monster of a city and metro. It's an economic powerhouse. Less than a handful of cities have bigger economies than Washington D.C. does. But it's also very close to Baltimore, and even though Baltimore is a good sized city itself, it loses a lot of counties to Washington D.C. because of DC's strength. Some of those counties may even be closer to Baltimore than to DC.