Nintendo and Sega used awesome compression that made their games look almost as good with one fifth the size
Lol, I don' t think so. Compare even Art Of Fighting, Fatal Fury 2, World Heroes or Samurai Showdown. These early games feature far more sprites on screen than either the MD or SNES could ever do. The reason for the increasing cart sizes was due to the level of animation and sprite detail, and only later on, more uncompressed sounds.
Now look at Last Blade and Garou, nothing on even CPSII pushed quite that amount of detail, and certainly not that level of animation.