HLS (hue, lightness, saturation) is one of several color measurement systems; RGB/CYM is another and all are valid representations. Consider HLS in which S (saturation) is the percentage of grey in the image with zero being pure color. It is evident that denser negatives, viz. ones that contain greater amounts of CYM dye components, have greater percentage of grey and therefore less color saturation. Sometimes thinking digitally is worthwhile even in the analog world.

I too normally slightly overexpose color negative films, for example rating Portra 160NC or Fuji 160S at 125. I find highly saturated colors too garish and prefer less of them; in fact, black & white is often the most pleasing representation.