Size vs Size On Disk

[budfudder]

I have a thumb drive that contains one (1) directory - that directory shows:
Size: 19.2 GB
Size on disk: 27.6 GB

Why is the size on disk so much bigger than the size? And how do I fix it?

 

[Grizzly]

What format are your drives ( FAT32, exFat, NTFS) ? What are the cluster size on each drive??

Here is an example: a file with the size of 25kB on a thumb drive with 4K cluster occupies actually 7 clusters. On a Hdd the cluster size is usually bigger, e.g. 8k. that means on the Hdd that file occupies 32kb in size instead of 28k on the thumb drive.
I hope this makes sense....