Using DOS to backup Flash Drive

I thought I would write a batch file to backup my flash drive. I used XCOPY with the /s switch, with the parameter being *.* It seemed to run properly, however, when comparing the size of the folder that contains the copied files from the flash drive, it is considerably smaller (about 100 MB out of 750 MB) than the amount of space taken up on my flash drive. Is this to be expected? Is there an easy way of determining what the difference is?

Thanks!