Huge variation in size of full backup

Re Acronis True Image Home 2011 Update 3 build 6942
The capacity of my local disk C is 931 GB of which I use only 66.4 GB (7%). I normally add or subtract very little data to the used portion so the size of used part of the disk does not change very much. Yet when I do a full manual backup of my C disk (I do a full backup of my C disk every few days) the “Total” amount varies by a huge amount. For instance today the total amount backed up was 94.1 GB. On October 16 (9 days ago) it was 62.1 GB, on October 7 it was only 27.5 GB, but on October 3 it was 61.0 GB.
How to explain this huge variation in the “Total” amount backed up?