Unity adds a lot of overhead. Looking at the graphics, this game probably would've come out to a handful of disks at the most, if it was actually made in the 90's. They would've used C, wrote their own engine specifically for the target platform, and the graphics would've been stored as paletted indexed formats instead of RGB (so, for a 16 colour game, 1/2 a byte per pixel, instead of 3 bytes per pixel).
I say a handful of disks, because games like Monkey Island came on a couple of disks, but this game has much larger sprites than those games did on average!
