By default, Apple's Time Machine software will grow the backup size to use all available disk space. To artificially limit this, do the following.
* Edit the file /etc/netatalk/AppleVolumes.default:On the line that begins '/var/hda/files/amahi-time-machine', add 'volsizelimit:XXXX' AFTER 'options:tm', where XXXX is the artificial limit in MiB.
* Execute, as root, 'service netatalk restart'