Copying files to an encrypted USB drive leads to major fillup

I have a Kingston Datatraveller Vault privacy edition with 8 GIG of space and I use a home-made program to copy images to it. Thing is, the amount of space needed for the images cab to be high as 3 开发者_高级运维times the actual size of the file while it's usually almost on par on a PC. It doesn't seem to affect other types of file though.

Does anyone knows why ?

Just so if someone has the same problem, I did find out the answer. It's because that particular key use 64K blocks instead of 4K and when you have a lot of small images, this is what happen.