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The cell size should be considered the "resolution" of the volumetric object. The parameter uses a real world unit of 1m. 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.
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