While not as svelte as MP3 players became, why are those early 2000s machines smaller than all the new models popping up amidst the retro audio craze ... why these newer cassette and CD players ...

This format gained popularity among consumers who wished to create cassette copies of their favorite CDs for portable or car audio systems without the need to split tracks across multiple tapes.

the audio cassette became the standard for music playback in vehicles. See 8-track tape. The tape is recorded in two directions, which doubles its length (two monaural or four stereo tracks).