|When you burn an "audio disc", the burning software converts the MP3 (compressed file) to CDA (uncompressed file) so the size of the file grows considerably. CDAs can be played in standard CD players such in a car, boombox, etc. An audio disc can hold approx 15-20 CDAs.|
MP3s are highly compressed. If you burn them as a data disc (rather than audio disc), they remain as MP3s & take up much less space. 100s of MP3s can fit on a single CD. These discs can NOT be played in a standard CD player, they must be played in a computer or a device designed to handle MP3s.
I'm guessing you burned 2 different types of discs, one audio (CDA) & one data (MP3).