You're entirely correct, that Knapp is undersized and underpowered when it comes to taking on the bigger and stronger ends, and especially the 3 technique tackles. Right now, he's holding his own using good technique and anticipation. He reminds me of how Ryan Harris was the starter at tackle as a true freshman, tipping the scales at 255 lbs (and that was a generous estimation).
However, Loren Landow is one of the best in the business, and I'm pretty sure he can add another 10-15 pounds of quality muscle in the off-season.