I've had the privilege of watching mature bucks making scrapes in the past. They don't always spend a lot of time on them. I've seen big bucks rub big trees and then just give a quick 3 or 4 kicks backward in the dirt and then move on. I don't think scrape size is as much of an indication of buck size as it is of the amount of time spent doing the actual "scraping". Small buck + extended scraping time = large scrape.