Predominantly used to describe women who, in any other setting outside of The State of Texas, would be considered slightly larger than the average woman. Not 'fat', but bigger. Women described as such often also have a 'large' personality as well as posses a penchant for rather exaggerated up-do hair styles.
Bob: Dude, what's up with your date? You usually go for the anorexic type!
Shawn: What do you mean? She's hot!
Bob: Dude, she's fat.
Shawn: Nah, she's Texas Thin.