There is a "prototype" size for a QB for a reason. That doesn't mean a guy has to be 6'4" 240 lbs to be successful, but all other things being equal, it is beneficial.
What do you believe the reason is that major programs recruit bigger guys, as a rule? You think they are all just morons? Is there some "height shaming" I haven't heard about?
Look, I don't believe that Tua's stature, in and of itself, is the determining factor in his success, but your conclusions go against a very large majority of the football world.
I would also point out that the rule changes have made it easier for a smaller player to survive the punishment. Prior to the emphasis on protecting QBs, and to some extent how DB aren't allowed contact anymore, a QB was driven into the ground multiple times per game, even when the got rid of the ball. A bigger player, on average, can better ansorb that punishment. That's nothing to do with "toughness", just physics.