Having trouble in understanding voltage doubler
The way this circuit works is that C1, in the steady state, is charged to Vp (positive on right side) during the negative swing.  This is because when Vi goes negative, D1 clamps the right side to ground.

When it swings positive to Vp, the voltage on the right side of C1 is 2Vp (or Vp+ - Vp- if asymmetrical.  That voltage is presented to D2 and current flows into C2 as required to balance the charges.

The actual size of the caps is irrelevant to this effect, it's just that C1 retains the voltage from the negative swing, and adds it to the positive swing.  The ratio between the caps only changes how quickly it arrives at that value.