Sounds like the shape of them is dicated by the internal geometry of the stnadard engine as Doug mentions above.

I guess to get really decent power from a zetec then you will need to shorten the crank throw slightly in order to allow you to maintain rod length whilst also moving the centre of the piston down so it can be dished with raised sides.

Obviously not the sort of thing you are trying to do, but thats probably the way to go for BIG power.