Rugby's most similar sport is football, and the exact opposite is true. I just find it extremely hard to believe that someone would rather be 11 pounds heavier with the same or slightly less strength than 5-6 pounds heavier and stronger. Again, you are also discounting the fact that strength training does add size as well. If you are playing at the professional level, then how are these guys not already advanced lifters with size? You say a big difference in squatting would not matter, but how is that even possible when you play a sport where leg drive is so crucial?
All parts being equal, the stronger athlete will always beat the weaker athlete whose only advantage is a small difference in weight. Take Aaron Donald of the St. Louis Rams who is arguably the best defensive tackle in all of football. He typically goes against guys up to 6"+ taller and 50+ pounds heavier and he absolutely dominates. Yes, his talent besides his strength is elite, but if what you said is true, "Once you reach a certain level, size trumps strength", then he wouldn't even stand a chance.
Strength coaches the world over (including the ones on this very site) prescribe strength training for athletes, not hypertrophy programs. Empirical evidence (like what I listed above) backs this up too. I'm sorry, but "I'm a professional rugby player and I say so" isn't a valid argument.