I think they need to bulk up for the week-to-week grind, not necessarily for specific opponents. The SEC and the Big Ten play a much more physical brand of football, week in and week out, than the PAC did or the ACC does. There's only so many talented really big guys out there, and those conferences get most of them. Teams just get worn down. Even teams like Nebraska and Iowa and Wisconsin have really big lines. If you ever played football as high up as high school, you know that there are times in the 4th quarter when you've just had enough. These smaller teams are having an entire season of "enough".
Dude, this isn't some kind of rocket-science discovery by me. Except for Oregon, which had the money and the framework to attack the portal and recruit 5-stars at will, the other 3 teams are all feeling it by now. I don't know that Stanford and Cal feel similarly beat up.