Timmy wasn't a big bloke by any stretch, but what makes him by a country mile the best 12 I've ever seen was that he had every single tool that a player in that position could have: the ability to run straight, a serviceable kicking game, rock solid defence, rapid acceleration, a step off at least one foot, vision and good hands. Sella had more or less the same package but in a 13 jersey (he played 12 as well). They would tend to lend weight to the argument that you don't need to be big, right?
Anthony Herbert always looked like a bruiser to me back in the day, but I don't honestly remember his height or weight. Daniel was bigger again, but he played in the pro era, when the athletes started getting larger.