Graininess, that is the appearance of grain, depends on
many things. The actual silver or dye particals which make
up the image are microscopic. In emulsion research they are
usually examined wtih an electron microscope although a high
power optical microscope will resolve them. What is seen as
grain is the "clumping" of grains at various levels in the
emulsion. Since the distribution of grains is random the
effect is to combine into larger groups of grains which are
also random. On prints the grain is actually from clear
areas in the negative where the grain clumps allow light to
pass.
   Graininess does vary with density. Usually visual
graininess is minimum at medium density and is more
noticable at low and high density. This may be what you are
seeing. Also, if you are getting grain on scanned negatives
but don't seen it visually it may be a sort of aliasing due
to a complex interference pattern between the grain and the
scanning pattern of the scanner.
   Note that some think that dye image films, whether color
of monochrome, have no grain. In fact they have the grain of
the silver image which generated the dye in development.
However, because the dye is in very small particals and
forms in "clouds" around the original silver, it may be less
noticable. This applies to the XP-2 which is a "chromogenic"
film, that is, the image is composed of dye particals rather
than metallic silver.
   Grain and graniness (percieved grain) has been
researched pretty thoroughly and articles can be found in
some of the older textbooks on photography. Kodak also had a
booklet on grain and graininess which may still be on their
web site. While this applies to color films it may still be
helpful. Search for E-58.
   As a rule, at least for B&W silver image films, exposure
should be minimised to that which will result in adequate
shadow detail. This tends to minimise grain and maximise
sharpness. For formats larger than 35mm, where minimum grain
and good avoiding image spread is less important somewhat
more exposure may result in better tone rendition.
   The ISO speeds for B&W still negative film reflect the
minimum exposure for good shadow detail. Color films and
other types of films are measured using a different standard
aimed for most films at best tone rendition.

When we first went hybrid at my paper, shoot film then scan
for prepress, we found that negatives that looked too thin
to print scanned well and scanned faster on our Leaf scanner
that did "normal" negatives.
darkroommike

Yes, in my book.

Briefly, the thinnest areas of the negative are hit by the least
light. Every emuslion contains a variety of grain sizes. The smaller
ones are the least sensitive. The largest ones are the most sensitive.
Larger ones have a statistically higher probability of being struck by
light. The lower the light intensity, the greater the proportion of
large grains that are exposed. Thus, shadows contain a higher
proportion of exposed and developed large grains than mid-tones or
highlights.