You are misunderstanding the adage and missing the point. Size implies strength which is finite and limited. You can always defeat strength with more strength. But skill is only limited by your commitment and at a certain point, the difference in skill does not afford a significant advantage, so that a fight between two fighters of near equal skill is really a toss up. Whereas, the outcome of a fight between two lug-heads slugging it out is a foregone conclusion - the big guy wins. In the context of how much skill and what kind of tactics, yes size does matter in the sense that you have to factor into your fight such things as the reach of your opponent, where to strike and what NOT to use. For example, if your opponent is taller and your high kick only reaches his chest it would be suicidal to use high kicks. If he has a longer reach, then you need to be even faster and careful in closing the range. In the real world, you don't get a choice of the size of your attacker. It's not like the UFC, MMA, boxing, etc. where there are weight classes. What are you going to use if you meet a mugger bigger than you? Strength or skill?