Dynamics are defined as the difference between loud and soft. That the greater the dynamic range the greater the magnitude of differences between the loudest and softest.
At least that's the official definition. In reality, we rarely come close to using anywhere near what is possible.
For example, the maximum dynamic range of a vinyl record (on a good day) hovers around 70dB while what's possible on a CD is just about 100dB. If we were to play a track where the lowest musical note was 1dB and the loudest at 70dB we'd not be impressed by the dynamic range. We'd not be impressed because our volume control would be set such that when the loudest note played the softest note would be inaudible. And, of course this would be even worse with a CD.
Technical issues aside, it isn't so much the magnitude of contrast that matters, but rather a more complicated set of rules that involves time as much as any other factor.
As we listen to softly recorded music our ears open up so we may better hear into the soft passages of music. Kind of like an automatic level control. Once opened, we're then startled when even a moderately loud passage comes rolling in.
Dynamics happen with a very set formula of time and contrast.
It's not the range of possible that matters, but how its implemented that offers us a sense of dynamics.
