This seems to be one of the main faults of Time Machine -- it just keeps backing up the same files over and over, and the size of the backup volume grows and grows.
This is one of the main reasons to try a "cloning-style" backup utility instead, such as CarbonCopyCloner or SuperDuper.
If you use a cloning app to create "a true clone" (that always represents the source drive "as it is" at the moment of backup), the disk space required on the backup volume won't endlessly keep growing as it does with TM. Instead, it will always be "the same size" as the source volume.
Both CCC and SuperDuper are free to download and try, if you wish.