Isn't Spriggan 6 stages? That would make the in-game content 3 megs total, but everything reused, from player sprites to effects and probably a fair ammount of code, would not take up the same space six times over in a single rom.
So a HuCard version of Spriggan with cinematics on the same level as Musha should be around 3 megs, but definitely no more than 4 megs with gratuitous sound samples.