In a compressional wave the denser the medium is at the compressions the smaller is amplitude?

We, let me think about that ...

I don't think I'd agree that you can characterize the amplitude of
a wave according to the density at only one point in it.  After all ...
a tiny wave in steel would be much denser at a compression than
a huge wave in air would be.

The amplitude of any wave is described as the difference between
a peak and the resting value.  Or even better ... half of the difference
between a maximum and a minimum.

So if you're looking at a longitudinal wave, like sound, I'd say if you
want to describe its amplitude, then you have to look at the density
at two points ... either the difference between the compression and
the resting densities, or the difference between the greatest compression
and the greatest rarefaction. 

That's my opinion.  I could be wrong.