More than likely its the cluster size thats different between the two drives.
If one drive is a 4k cluster and the other drive is say an 8k cluster then the same number of files will use up more space on the 8k drive.
Reason for this is a 1k file will use 1 full cluster, 4k.
So it would be 1k used in files but 4k total on disk. Hence the 8k cluster drive would use a full 8k to store that 1k file so you would be using 8k.
only one file can use a cluster at a time. Thats why if your using a lot of very small files a small cluster size is best to lessen lost disk space.
Bif files :ie 1mb or more, a larger cluster size is better.
Note: mentioned cluster sizez may not be possible on actual drives. just for reference