A little math explains audio CD capacity:
Audio CDs are digitized at 44.1E+3 samples per second, two bytes per sample, each of 2 stereo channels. That works out to 176400 bytes per second. A typical 45 minute CD, therefore, requires 45 X 60 X 176400 bytes, or about 475 MB. A normal CD capacity is 650 MB. Audio CDs were invented before digital compression techniques existed, so the audio data is stored 'as-is', taking the full uncompressed storage space. MP3 compression will typically compress to about 10:1 ratio.