"Quality" is probably not a good term to use here, at least for now.  Smaller Irradiance Sample Size means that, when a "gob" (a silly term but I think it's fair) of Indirect Lighting illumination is painted over part of an image, it will be a small "gob".  A large value will give you large "gobs".  The gobs of illumination are averaged together.  Large gobs overlap each other a lot, both in area and in the number of overlaps, meaning the averaging will tend to be very smooth, but less accurate (e.g. try painting a picture with a 500 pixel brush in an image editor).  Small gobs only overlap each other a small amount, and less often, meaning that the averaging will be more accurate (which is good) but also more prone to showing an artifact where a difference in brightness between neighboring gobs is too great (which is not good).

This may be a completely silly analogy but I think it's in the neighborhood.  I feel that "Smooth and less accurate" is better than "Full of artifacts and very accurate".  It would be great to have "Smooth and very accurate" but I think that will have to wait for SR1 or later.

My Freebies