Database backups are roughly the size of you data, not the size of you database files. This is assuming no compression. If the .BAK file is still getting bigger it's either because the amount of data is growing (despite the deletes) or possibly (but unlikely) you have the backup doing an append. If this is something like an electricity utility getting meter readings every minute could be adding more records than your delete process is removing. Consider using partitioned tables to split the data a bit, then you can just drop one of the partitions instead of doing an expensive and slow delete.

Leo