I highly, HIGHLY doubt that a HuCard game that is 2 megs would take up 6 megs of space on the Genesis. In fact when I see a 4 meg game on the Turbo, it seems like a 4 meg game on the Genesis in terms of overall game size, capabilities, features, etc.
Also, doesn't RISC stand for Reduced Instruction Set Computer or similar? Reduced instruction set. That means reduced instructions to accomplish the same tasks. But it really doesn't matter, as the 68000 was CISC based... not RISC.
When it comes to code, the 680x0 code will be larger than the 65x02 and 65816 and even it's younger brother, the 6809. And the 68000 opcodes are RISC type in format, not 100% because a few opcode combinations are not possible - they're simulated on the assembler side. Also, any sort of array or data structure that is not a even multiple of 2 bytes, really throws a wrench into the speed of accessing it - usually padded to 16 or 32bit for simplicity or speed, at the expense of size. Trying to turn a disadvantage into a great sales pitch?
Anyway, I remember looking at SF2:CE for PCE and all the characters (frames) were uncompressed graphics. The backgrounds are compressed though (maybe someday I'll write and extraction app for that). The hucard only has access to 8k of ram, so decompressing a bunch of frames into ram for quick access is not an option (snes has 128k and the gens 64k).
I just noticed something about the genesis version, it's running in 256 x 2XX mode. That's kind of strange. They must have done that to save space on graphics. The arcade is 384x224. The PCE could've came really close with it's 380pixel mode(mid res), and the gens with it's 320 pixel mode, but the snes is limited to 256 (the SNES 512 pixel mode is pretty much useless because of the sprite/scanline limit). I haven't played the other Gens versions. I wonder if they run in 320 or not.
Btw, I thought the genesis music was cool and impressive considering the limited number of sounding instruments you have on that yamaha chip. The pce music impressed me more though as the samples sound great for trying to emulate the arcade's instruments. The snes version is least impressive for what it's capable of. Also, the gens isn't known for it's digitized sound playback (most sound effects in genesis games are synth - instruments from it's lib) so the quality of the digitized effects was about what I was expecting.