No they shouldn't be expanding... I would say maybe you are either misjudging the actual size of the files, or there are files you aren't aware of on there.
You know a lot about everything so when I put files on to a disk formatted like a usb drive are they supposed to expand?? It's weird. I put a bunch of stuff, primarily mp3's onto a standard dvd r disk and it was 2GB but when I put them on, the disc couldn't handle it. The dvd r was formatted to be 4.5 GB. Also when I put the files on to the disc by selecting them all it wouldn't work so I had to put them on in 3's and it managed to work, just, but the 2GB I put on to the disc took up the entire thing