maybe youre right. I also kind of remember that there are short load times before the bosses? have to check on that again.
did you also include the additional available 64KB from the CD-ROMS ADPCM memory, which can be used for graphics as well? Thye might have used some of that RAM too. then it could go up to something around 8Mbit easily.
anyhow, only looking into the software would tell us how much in gameplay code and graphics was exactly used.
Your right, it has 64kb +64kb each level or load, even if it is only used for sound, that frees up the main memory for only code and graphics.....the 4 megs in musha has to be used for mundane things such as ending credits, title screen, the very nice looking opening cutscenes, the ending scenes, music, sound fx, code and ingame graphics, so you start to see why it looks more impressive when compared to other 16bit vertical scrolling shooters, on cartridge or hucard format. Just look at the first level alone, the floating crystal city and then the floating castle with the green lightning in the BG, the variation in BG tiles and sprites is fantastic.
To think that Spriggan isn't even optimized, what I mean by optimized is two loads per level that don't hurt the flow of the game, the only CDROM2 game that comes to mind right now that is optimized,(two loads plus Super CD compatible) is Macross 2036, in my opinion this full potential is not always fully realized, one level that is though has you starting out in space where you pass a moon then come to a large space fortress/craft, then your inside the fortress, then you have a separate back ground not one but two large bosses plus your own ship is a completely different sprite, after defeating the two bosses you go to yet another unique back ground, where you fight another huge boss, with beautiful animation, the its attacks are very well done, on top of that you have several very, very slightly animated pictures that pop up during the level...name one horizontal hucard shooter that's any where near as impressive as this and when you factor in the ADPCM sound fx which are amazing, it becomes even more impressive.