Time Machine Backups taking too much space? Yesterday, I installed an external hard disk for my Time Machine — well, a 1.6Tb partition of a 2Tb hard disk. I am running a new iMac with 512Gb SSD. The internal drive is about half full.

After a day, I have about 5 backups, and I find that I only have 448Gb available.

I am still trying to get the other folder sizes (understandably, this is taking a long time), but the first backup folder is about 250Gb. At a rough estimate, this suggests that the subsequent backups are about the same size. At this rate, I will run out by tomorrow.

I understood that the Time Machine backups are sort of differential, and that subsequent backups are mostly links to the original backup files.

The question is: is the size, being mis-reported, or is that a correct estimate of the space used?
I would probably do two things: 


Let time machine run until it fills up the 1.6 TB and then see how long a windows you have. macOS will clean up the destination - purging old files as needed. 
Use tmutil compare or a tool like Backup Loupe to check what is taking the space. Other tools do a lousy job of showing you actual space usage in time machine. 


Why does Time Machine estimate a 109GB backup size, but then need 235 GB?
Time Machine - List of Files that were backed up