After some cursory searching, I found a forum post pointing to
compact verb. From the
scans the bands of a sparse (SPARSE or
SPARSEBUNDLE) disk image containing an
HFS filesystem, removing those parts
of the image which are no longer being
used by the filesystem. Depending on
the location of files in the hosted
filesystem, compact may or may not
shrink the image. For SPARSEBUNDLE
images, completely unused band files
are simply removed.
hdiutil compact drive.sparseimage and it successfully reclaimed almost 98% of the space.
(I guess it's one of those days... I should really learn to Google stuff first).
Edit: I tested
compact on a 1GB
sparseimage with just a few text files, and it ran quite quickly, but my Mac is taking quite a while to compact the 90GB image. Be prepared to wait.