And this is the crucial insight. An image, or a game, does not contain all its data in those bytes you have stored in its file. A lot of information is spread around in the environment. It's one of the main ways demo-makers can cram so much content into so little space - they pull as much stuff from the environment as it's possible (and allowed by contest rules).

To give an example: one of the most basic tricks, if you're allowed to use OpenGL and WinAPI, is to use WGL to render your system fonts into 3D shapes[0]. This allows you to get a lot of 3D models from just the few bytes it takes to call wglUseFontOutlines()[1]. You may think that there isn't anything interesting in a basic font, but - even forgetting about Unicode - every Windows has Wingdings...

(Or look into the good old .EMF[2] format, which was basically direct encoding of Windows GDI calls.)

For much more in-depth discussion on where exactly information is stored, be sure to read GEB[3], if you haven't already.

[0] - https://www.youtube.com/watch?v=iri5CgLglkk - 1/10th size of Mario!

[1] - https://msdn.microsoft.com/en-us/library/windows/desktop/dd3...

[2] - https://en.wikipedia.org/wiki/Windows_Metafile

[3] - https://en.wikipedia.org/wiki/G%C3%B6del,_Escher,_Bach