Output compression

I’ve tried a few different output formats for animations (both Blender Render and Cycles), varying the compression percent, but it seems to have no effect: 0%, 50%, and 100% all produce files of the exact same size and similar appearing image when played. Any idea what i might be doing wrong? Thanks.

What file format ?
High compression does not have to mean low quality.
Some formats such as .png is lossless so final quality remains the same but compression can change the file size.
Some like .jpg is lossy so low quality setting is lower quality of final result but smaller file size