1. Smaller graphics, and less of them. That means doing things tile-based instead of having whole-screen sized backgrounds, and re-using tiles flipped and rotated, things like that. It also could easily mean doing recolors of sprites for enemies, either using palette swapping shaders, or just using grayscale blending using image_blend.
2. Smaller audio...this also means using less audio, and shorter audio, and compressing said audio to lower quality values(especially for music, sound effects typically don't help to compress much unless you just have lots of them which is counter-productive to the topic at hand). Like sprite recoloring, you can also at run-time do variation of sound pitch and volume, randomly if you choose, to help get some variety out of a single sound effect.
Any thing else, like objects, code, room amounts....those take up very little actual space for the most part, and generally aren't worth trying to optimize. The only exception would be if you are using code to store data, like if you stored 3d model vertices in code or something, making extremely long code files.