linux - What causes files to use up more more disk space than the file size suggests?

du was showing some drives with much less space than I was expecting, and ls -alh also showed the sum at the top to be a factor of three more than the sum of the individual files. Following this answer, I checked with ls -s, and sure enough, most of the files are using three times as much disk space as their size. What causes this and can I do anything to get the disk usage down?

Edit

I'm seeing output like this from ls -alhs:

 50K -rw-------   1 xxx xxx 9.0K Jan 29 20:34 20120103.gz
242K -rw-------   1 xxx xxx  67K Jan 29 20:53 20121130.gz

so the problem isn't that my file sizes are much less than 4k.