Some of that just comes down to how positions are grouped on the rosters, and the fact that we have a lot of walk-ons that inevitably end up at DL because they aren't athletic enough to go anywhere else. Most of the sub-275 guys are edge players, which Oregon lists as OLBs and also don't have any 275+. A few young guys are also significantly heavier than their currently listed weight at this point.
But all in all I agree size is helpful (and Oregon has comparable size to Big Ten teams), but not everything. I do think the grind of Big Ten play will get to Oregon more than they think, but a lot less than it's going to impact USC/UCLA.