I've wondered that, myself. I watched a little of the Louisville game the other night and it looked like they were wearing their pads on the outside. Maybe just my tv, though.
back in the day the lineman were not allowed to use their hands to block. You used a forearm with the elbow up and out and the shoulder pad. The larger shoulder pad gave them more blocking area. Now the linemen use their hands and stop the momentum of the person they are blocking by punching with their hands and using their legs. so the smaller pads give them more freedom to move their arms. Receivers and quarterbacks always used smaller pads to have more freedom.