How to make sprite animation when the sprites are of different sizes?

I have to make an animation of a water drop getting converted from its drop shape to that of flowing water. I have all the required sprites as different png images. But the problem is that their sizes are different, obviously. So if I keep the pivot point in their centre then the position of the sprite changes during animation i.e. it gradually comes down during animation instead of remaining at its original position. If I manually set the pivot point then I am not able to get the same point for every sprite and so it gives a shaking effect. Is there an easy way to do this?