Two very different things called compression
If you work with music, "compression" means dynamic range compression: a processor that pulls loud peaks down and lifts quiet passages, so the whole thing sits at a more even level. That is a creative tool and it changes how the audio sounds.
This page does the other kind. An audio compressor in the file-size sense re-encodes the audio at a lower bitrate so it occupies fewer bytes. Loudness, dynamics and level are untouched. It is worth being explicit about, because searching for one and finding the other wastes everyone's afternoon.
Nearly everyone who wants to compress audio file by file is trying to clear a limit: an email attachment cap, a submission form, a podcast host's ceiling, a messaging app that refuses anything over a few megabytes. That is the job an audio compressor of this kind is for.
Where the size actually goes
Bitrate is the single lever, and it is close to linear: halve the bitrate and you roughly halve the file. Two other things matter almost as much, though, and neither is a quality setting.
Duration is the obvious one. Nothing reduces a file like removing the parts you do not need, and a trim costs no fidelity at all.
Channel count is the underrated one. A great deal of audio that is stored in stereo is functionally mono: a single voice, a phone recording, a screen capture, an interview into one microphone. Converting that to mono halves the data and loses nothing that was ever there.
What happens to lossless sources
A WAV or a FLAC has no bitrate to lower, because they store the samples rather than an approximation of them. To reduce audio file size in those cases means choosing a lossy codec, so the output becomes an MP3 at the bitrate you picked. That is the honest behaviour, and it is what you wanted if you came here to make a WAV manageable.
For lossy sources, the format is kept: an MP3 comes back as an MP3 and an M4A as an M4A, so nothing downstream has to change. The list above shows the before and after size for every file, so you can see whether the trade was worth it before you download.
Everything runs in your browser using ffmpeg compiled to WebAssembly, cached after the first file.