To add to the audio compression: it isn’t possible to further compress an mp3 file without losing any quality. You can either:
Recompress to a lossy codec (mp3, aac, opus). This will lead to smaller file sizes if you set the bitrate lower than that of the input file, but it will always worsen the quality, no matter the bitrate.
Recompress to a lossless format (flac easily being the best one). Going from a lossy to a lossless format will increase the file size (sometimes by quite a substantial amount), while keeping the same quality. There is very little reason for you to do this
keep the original files (my recommendation)
If you’re willing to spend some extra time learning about audio compression, you can download lossless files and compress those directly to whatever format and bitrate you want. The quality will be better than option 1 above, as the audio is only lossely compressed once instead of twice.