I would think it depends on how you harvest the curds and drain them before pressing. Something soft like a brie which is ladled from cubes in pot to mold will decrease plenty in the mold, while something like a gouda which is drained in the post most way will decrease far less.
A cheddar done with the traditional english cheddaring/layering shouldn't decrease much at all in the mold, since most whey will already be gone my molding. If you are doing a cheddar any other way you could have more moisture. Cheddar is also pressed fairly heavy for a good amount of time, so it will decrease some.
I've never made one, but will in a few days. I can't imagine a reduction of over 20%, but others here would know better. You might want to post your process you plan to use and maybe someone having done that technique could be very specific.