If you want to see how compressed your music files actually are you can use software like spek or ffmpeg to visualize the audio data.
This is the spectrogram of a flac file with a CD as the source:
flac1242668 90.6 KB
And this is the samg song converted to 128kbs mp3.
mp31242668 87.4 KB
To save space, frequencies that humans cannot hear are cut off as part of the compression. This data is lost permanently.
If I were to convert this mp3 back to flac and then created a new spectrogram, it would still look exactly like the 2nd one. Which is why serious collectors often check the spectrograms of audio files instead of believing what the audio extension suggests to detect transcodes.