Time Machine only performs full back-ups, no incremental back-ups?

I back up my Mac Mini to an external partitioned 2 TB USB drive, with 1.5TB and 0.5TB partions. The back-up portion of the drive used by Time Machine is the1.5 TB parttion. Time machine also backs up the 0.5 partition of the same external drive to the 1.5TB partition. The total size of the data on the Mini drive is about 60 GB, and the data in the 0.5 TB partion is about 240 GB, so the size of the first full back up should be about 300GB. Within the space of about 8 hours, Time Machine has made three back-ups that are using a total of about 980 GB, or about half of the 1.5 TB partition. I opened up the back-ups folder and it appears that each of the three internal folders for each back-up contain back-ups of both drives (mini and .5 TB partion) as a full back-up, not an incrememntal. At this rate, the 1.5 GB partition will be full in 2 days! Any thoughts or ideas? Thank you in advance for your kind replies and advice.