It doesn’t seem like the iDisk disk images get compacted automatically to take less space if you delete files, so what I learned from the bug is generally useful.
You can compact a “sparse” disk image manually from the Terminal application.
The iDisk can’t be in use, so close any files opened on it. Then open a terminal and run:
hdiutil unmount ~/Library/FileSync/*/*.sparsebundle hdiutil compact ~/Library/FileSync/*/*.sparsebundle
hdiutil compact feature also works on old fashioned sparse disk images (all data in one file versus the “sparsebundles” that are really directories with lots of smaller files). I used it to reclaim a few gigabytes from some old disk images, yay.