At all but the shortest (well under a minute) efforts leg strength isn't important. For minutes you're limited by how much oxygen you can get to your muscles, and after that lactic acid accumulation determines how hard you can go.

Bradley Wiggins was looking spindly on his way to win the 2012 Tour de France; and Marco Pantani was dimunitive when he won in 1998:



When you look at pedaling forces they're not that high. The blue circles are a 1.5 mile long 7% grade. Average forces don't exceed 275 Newtons which is 56 pounds. Pedaling forces are sinusoidal with a peak approximately 1.4 times the average magnitude making peak pedal pressure 80 pounds. The red curve is the 185W average for the effort. The cursor is centered at 75 RPM (everything left is proportionally slower) and 185W.