Why does iTunes keep converting my high quality mp3 files to low quality?

For some reason, iTunes keeps converting my 256k and 320k mp3 files into 128k files. When I search for the file on my hard drive and view properties, I find that the file has gotten considerably smaller (for instance, a 9 MB file has somehow turned into a 3 MB file). This has happened to files from a variety of sources, including songs I import from CD’s and download from the web. I’ve got iTunes set to import tracks at 256k, so I know that is not the problem. Can someone please tell me how to solve this problem? Thanks in advance for your help.