Given Irie is clearly G10 material, and we see some ridiculously tall guys in the G10, I see nothing wrong with him saying he's too short compared to the guys in G10 that ought to be playing basketball.
And being tall does have its advantage in tennis. That particular scene shows a lob Irie he initially can't reach, so the implication is pretty clear that if he was taller he can reach the ball. Of course Irie reached the ball anyway because it's just an act, but there's nothing wrong with the assumption of 'tall = greater reach = advantage'.