Your logic is not correct. Powder and people are not the same. Volume is volume, and particles of fire retardant are not changing size or quantity. If pressure drops substantially in an extinguisher, it’s leaking.
A can of fresh spray paint works because the seals are all still good and the nozzle isn’t clogged, and cans of used spray paint can leak out pressure or have the nozzle or other plumbing jammed up with dried spray paint.