Houston always has some kind of Oil and Gas convention. For example, there was a Oil & Gas technology related convention in Houston last Thursday and Friday. I am pretty sure there was something going on in the city when you flew few weeks ago. You probably just picked a bad day to fly from IAH-SFO.

You are right, IAH -SFO flight is always packed. I fly UA to Asia often and that flight is the only UA non-stop flight from IAH to West Coast for Asia flight connection.

As far as larger plane, IAH terminal A can handle UA's 757 and I believe the UA will sell out seats on 757 just as well.