The hydrostatic pressure formula (P=p.g.h) is used (eg) to calculate the pressure at the bottom of a container. Now this pressure is equal independent of the mass in it as long as the bottum surface is equal. This is because it depends on the local equilibrium of forces, it cannot depend on what's going on far in other parts of the container. The extra mass is contained by the vertical walls of a container. See Why the pressure in liquids only depends on the height?
Now often this is demonstrated with water(molecules). But how big can these molecules become to keep the formula valid for it. In the following (poor) picture are the 'molecules' touched by two other molecules. So the force on it would be higher on the bottumsurface. In this picture however I have exagerated the shape of the molecules and the walls aren't uses. But imagine a substance that touches the walls (like watermolecules do) but the also have a slightly other shape. To what shape/size is this formula valid. Or could even a substance with tennisballs have the same effect?