For sound Godot currently supports OGG and WAV. The problem is that the OGG version of a 20 MB WAV might be around 500 KB.
This means that games using WAV files are considerably larger than those using OGG. This will easily get above our upload size threshold and even if we had no threshold, the bigger size means more for players to download, which is bad.
This feature means that we automatically compress WAV files into OGG at upload time, thus saving considerable amounts of space.
While the big culprit usually is the sound files we will also compress textures (while making sure that the compression is not noticable!).