To an extent, size does make a big difference. For example, I consider myself a strong 160lber, and when I roll or wrestle with guys that weigh 140, sometime 135, I just dominate them like nothing. I don't use fancy technique on them, it's just overpowering them basically. But also, when I'm going up against guys that are heavier than me, it IS harder to get certain positioning, and to keep it, but it also makes you work that much more on your technique, to offset any size disadvantage you may have.