I agree with Merrypransker. Grappling arts focus on leverage rather than pure strength. In some grappling tournaments we even have an absolute devision where a grappler could try his skills against grapplers in higher weight classes, and vice versa.
As for arts like Muay Thai, well those are about timing, distance and accuracy. The power of kicks, and punches also comes from leverage, speed, and physics of body movement. As for stregth in and of itself, it's important for all martial artists. Those who say that size and stregnth doesn't matter in a fight, are fooling themselves. When all else is equal, strength could be the deciding factor between a winner and a loser.
MA fanatic