It's absolutely a zoning issue. The Texas legislature is literally overriding large cities to force them to allow smaller lot sizes. Most of Dallas, for example, is R-7.5, which drives up cost because land is expensive and it requires you to dedicate a lot of it to your yard (max 45% coverage). Only a few areas, like some PDs in old east Dallas, have been rezoned to allow the smaller lots, though you seem to believe its all of Dallas city limits.