Every time Time Machine does a backup of my Yosemite working disk (140 GB), it adds about 20 GB to the size of the contents of my backup disk, and it fills up very quickly. This happens even during the night when both I and my computer are sleeping. I don't think it should work this way!
You can use a program like BackupLoupe to browse the changes added in each backup version.
Otherwise, the tmutil command (specifically
tmutil compare) can be used to observe the differences between two TimeMachine backups (or a backup and the current state).