OK, I vote for "sparsity".
This overloads the word, since it also applies to (low) degree of density,
but it intuitively fits, sparsity in our new sense growing with volume.
This word also comes to us via the Latin from proto- Indo- Eurpean, but got
a little more battered in the process, from "(s)preg-" to "spargere" to
"sparse".
Honk up the Working Group.
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Whatever happened to plain Indo-European?