I have a very limited understanding of FM synth but from what I've read tracks like these would take up more space on a cartridge, better sound = more space. Which is why a combination of the cost of cart size and limited sound tools gave the Genesis its reputation as a inferior sounding system vs the SNES.
Hmm, I'm not sure. AFAIK, the vast majority of Genesis soundtracks were programmed, as opposed to samples. The cart would have what boils down to a player piano music roll and the OPN chip would use that data to trigger sounds across different channels. The level of sound quality wouldn't vary, since the only thing on the cart is raw data rather than audio files; the sounds themselves are being generated by the audio chip. Think along the lines of the MIDI music in FF VII vs. the Redbook audio in sports games for PlayStation. (I'd imagine most of the speech like in Sonic 3, NBA Jam, Mortal Kombat, etc. would be sample based and therefore would take up a lot of storage space.)
EDIT: I'd also guess that the reason these songs sound so clean is because they're using emulated hardware. There are software plugins for DAWs that emulate specific vintage sound chips, so you can quickly program MIDI tracks. If these really were programmed by hand, like in LSDJ for GameBoy, I'd be surprised and super impressed.