This isn't an eight bit game, that executable is a Win32 binary.
Shame for misrepresenting as an 8-bit game, shame for making it Windows only (where's the linux version? where's the source code?), shame for that bullshit download link where you have to sign up for some phishing site to get the game.
The game is designed to look like a game for an 8-bit console (NES etc.). The chip tunes and sound effects are sampled or generated in software, not created by an actual sound generator chip, since your computer doesn't have one. The game probably runs in true color, too. Don't think you'd want an 8-bit executable, it would be really hard to make that work on modern systems.
21 megs?
the original doom shareware was 2 megs....
whats taking up so much space?
Bloated code, unoptimized graphics (much of it is probably huge bitmaps instead of the clever designs using tiny, repeated tiles an actual 8-bit game would have used, sampled sound). A NES cartridges held around 512KB. The 5.25" disks used with the Commodore 64/1541 drive held 170KB, or 2x170KB if they were double-sided.