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The cell size should be considered as the "resolution" of the volumetric object. The smaller the cell size, the finer the resolution. Just as a bitmap image can reveal more detail when zoomed in if it contains a lot of pixels. The cell size parameter is necessary in order to keep the RAM usage under control. So the final RAM consumption is not influenced by the number of particles in your simulation, but instead on the number of cells which are created at render time, which define the resolution of the volumetric object.
How cell density is determined
It is important to understand that what you finally see in the render is the contribution of the density of each cell, NOT particles directly turned into density. So how is the amount of density for each cell determined? Each cell looks from it's corners into the cell to see how far away a particle is in regards to each corner. Depending on the particle radius, density multiplier, and max final density - some density may be shown, or nothing at all.
This illustration shows one particle of a certain radius, and a cell of a certain size.