Typical Democrat spin. There was no "middle out" growth in San Antonio - or anywhere else. According to Forbes and even San Antonio's own Economic Foundation, the growth was primarily spurred by bringing in new business (you know, the "top down" folks). And business came in because the Republicans that run the state of Texas have provided great economic incentives, such as lower taxes for corporations (which Democrats oppose).
So it would seem that San Antonio has grown not because of Julian Castro's meager efforts, but in spite of them. Democrats want higher taxes on the wealthy and corporations, but Texas Republicans have kept taxes low, thereby bringing in the businesses that employ people and strengthen the middle class. The fact that the middle class is growing is simply because there are more wealthy businesses that are hiring people. And Texas Republicans are the ones who are responsible for that.
If the Democratic Natuional Convention wants to be honest and choose a keynote speaker most responsible for the growth and success of San Antonio - and most of Texas - perhaps they should choose Republican Governor Perry...