Acronis 2017 backup speed and size

I have two systems basically doing the same thing.

The local (C) drive size (OS Win-10) displays 22 gig of used space and once done with the sector by sector backup onto an external 4TB drive, that drive displays 921 gig of used space.

I turned off the "back up unallocated space', to see if that would make any difference, but it didn't.

Why am I not seeing something close to the same size of the C drive on my external drive, which has only one file...a full backup?

Any suggestions out there?

Thanks