First of all toughness and bravery are two different things altogether. Keane was 5ft11 and Gattuso was 5ft10 and they would have gone through a brick wall for their team. Secondly as said, goals from headers depend on many aspects including
a- timing
b- tactics
c- delivery
d- players around you
Dwight Yorke was 5ft10 and was not a particularly tough player but he scored a lot of headers. The treble side was not particularly tall. Out of the first team only Stam, Johnsen and Schmeichel were beyond the 6ft mark. Schmeichel was a GK, Teddy was a squad player, Beckham (6ft) was the corner taker which means that Stam and Johnsen were double marked in terms of corners. The 2008 CL winner squad had VDS, Rio, Wes, Carrick and Ronaldo over the 6ft mark with the likes of Oshea and Fletcher on the bench. Football had also change throughout time thus restricting what defenders can do to stop a player from scoring.
Stam was a machine. His physical frame was imposing, he rarely ever committed any mistakes, he was comfortable with the ball and his recovery was amazing considering his size. Unlike Vidic (ie Fernando Torres) I can't think of any striker who could claim Stam's scalp on a regular basis. The closest thing I can think of is Pippo Inzaghi. There again, we're talking of a players whose performances were more akin of winning Oscars for best performances then Balon d'ors