What the MP3 costs you
WAV is a thin container around raw PCM: nothing is modelled, nothing is predicted, every sample is written down as it was captured. At the usual 16-bit, 44.1 kHz, stereo settings that works out to 1,411 kbit/s, or roughly 10 MB per minute. MP3 keeps a psychoacoustic model of what an ear is likely to notice and discards the rest, so at this page's default of 192 kbit/s a minute costs about 1.4 MB — one seventh of the original.
What disappears is mostly sound that was masked by something louder nearby, plus the top of the spectrum. Encoding a white-noise file with exactly the arguments this page uses, everything above roughly 19 kHz was gone at 192 kbit/s, and everything above roughly 17 kHz at 128. Through normal speakers, on a finished mix, that is hard to hear. On a sparse recording, a solo cymbal or anything that will be encoded again afterwards, it is not.