It would also depend to some extent upon how tightly it's packed in the first place, along with how forcefully and quickly you compress the box.
If it's just poured in loosely, there'll be a lot of air spaces. In that case, slow compression might actually lower the surface level as the material settles into a more compact arrangement. Conversely, a rapid compression could cause the air to blow the stuff out.
If it's been tamped down so as to leave little air space, a slow compression would probably cause a bulge on top as DH said, but a fast squeeze might 'squirt' it out.
Why don't you get yourself a plastic box (like Gladware or similar) and some different types of powder. Experiment, already!