Why is crankshaft diameter bigger than camshaft?

1- Because of the massive load on the crankshaft due to piston's up and down movement .

2- Because the huge amount of twisting torque . 

3-Because the timing " Rotates once for each two cycles of camshaft " .

Because the loading on the crankshaft is much higher than on the camshaft.

Crankshaft is bigger than camshaft because it holds the connecting rod since camshaft holds the rocker arm

due to crankshaft revolving at twice the speed of camshaft.