A small experiment:

Using DBPoweramp, if you convert an extremely compressed file (say WMA 48 bps) to WMA lossless, you'll find that file size has increased tremendously along with bitrate. How does that make sense? Isn't a lossless codec supposed to simply zip the file? In my book I should have gotten the exact same file with the exact same bitrate.

When you compress a lossy (wma) file to a lossless format, it is decoded, and only then recompressed. Lossless codec isn't aware that the original was encoded by a lossy codec, therefore it uses its own algorithms to shrink it.

Transcoding from lossy to lossless will almost always produce a file which is bigger than the source, and is therefore not a good idea™.

When you convert a 48kbps WMA file to WMA lossless, it first gets decoded to WAV. This will cause the file to get almost 30 times bigger. Then, it will compress this file with WMA lossless, reducing the size to maybe 50 or 60 % of the WAV file. So it's quite normal that the resulting file gets about 15 times bigger than the original WMA file.

Edit: I assume you mean 48 kbps instead of 48 bps. If you really have a 48 bps file, then I'm very impressed, but I wouldn't want to listen to it.

Just remember that the 'zipping' process, or lossless compression algorithms, always work on wave files only.

Edit: This thread should belong in the 'Other audio codecs' sub-forum.

Sorry for not paying attention to the forum on which I posted my question and thanks for all answers, especially to Poisondan who corrected my measurement unit.