That is a new one to me.. If you didn't lock the sparsebundle size.. which is possible. then I suspect something has gone wrong. If you want to save your old backups archive off the backup.. not the sparsebundle.
Erase the disk in the TC via the airport utility. Setup a new backup and copy your backup to it.. this is a bit tricky.. see Q18 for the method.
I think you will do better just saving the archive and start over with fresh TM backup. Do not load anything else onto the TC.