If you watch during the middle of any given season, most of the time that a top 25 team is at home they are rarely an underdog by more than four. Exception is when they host one of the top two or three teams that might be dominant.
You will often see the lower rated team as the favorite against the higher ranked team if the former is at home. For example, if the game were coming up first, I would think that UT would be a favorite against Florida, even though Florida is higher ranked.
I can't give you the numbers, but over time it certainly seems that home field advantage in college football is a huge factor in setting point spreads because, over time, that one factor seems to account for more "upsets" than any other. I assure you that if Cal were hosting UT, the line would be more like Cal - 6, which is a big difference from Cal +2.