What constitues as a "big advantage"? Foreman had 3 pounds on Frazier, 4 pounds on Ali, 6 pounds on Lyle, and 12 pounds on Norton. I wouldn't say any of those are really big disparities in size when it comes to heavyweights. It's not comparable to say Bowe-Holyfield or Klitschko-Haye where Bowe/Klitschko had up to 30 pounds on their opponents.
You're choosing specific opponents though. I don't have time to look up specifics right now, but I have looked at it in detail in the past, and Ali had a larger average advantage in weight over his opponent than Wlad did. Foreman also.