Liquids (and solids) are much less compressible than gases, but they are still compressible. Constant volume (incompressibikity) is just (sometimes) a useful simplifying assumption. (In other contexts like sound/seismic wave speed, it would be, well, complicating to say the least, given that would result in an infinite wave speed.)

If you have a tall enough, a column of metal, or even rock, it will deform under the pressure from its own weight. A penny is just very small and light, and deformation is negligible. Like a solid, a liquid will also not deform without without some force being applied, but the type of deformation is different.