While not being an expert on the matter, I can’t see that the actual code running your game would provide the very much weight to your game whatsoever, I would instead look at the usual suspects (video, audio, models, textures). Otherwise I think it’s all dependent on context, but I’ll pass on a few things that have helped me.
Video
For video you want to make sure, firstly, that it’s absolutely vital to your project, then see if you can’t reduce the quality/fidelity or the resolution. For example, if you are using a high quality video as a moving texture for your main menu, that’s cool, but if your game’s setting/theme is of mystery then you can thematically afford to reduce the resolution by half (while using it over the same area of space) and then add blur effects to mask any artifacting. Resolution and size greatly decrease but it still plays into what you’re doing, thematically.
I remember I once was in a situation where our team’s logo splash animation was 15 seconds but weighed in at around 10MB and that was simply not acceptable, so we cut it down greatly.
Sound
For sound effects, again, make sure that you haven’t put in too many different sound effects and that your musical tracks aren’t overly long or complex.
Look to see that the bitrate for your sounds isn’t too high, your average feedback sounds can go as low as 128kbps without generally losing effect. Depending on the complexity of the music, you might like to keep it around 192kbps, but look to see if you can’t go lower, at all.
If you’re using WAV, then get your hands on Audacity (http://audacity.sourceforge.net/) then convert any WAVs to MP3 and that will greatly help.
Also, are you using too many different musical tracks? Games are given great leeway in that you can be clever with music or recycle it a great deal without complaint (so long as your music stands up in it’s own merit).
Images (textures, gui images, et al)
Are you using PNGs when you could be using a lossy format, such as .JPG? Are the resolutions of the images necessary?
If you are putting in large textures then changing the actual dimensions of them in Unity I don’t believe it will actually cut down the size at all, so, generally speaking you would want whatever you make outside of unity to be put into it with the least amount of changes (Don’t make a 1024x1024 image in Photoshop, bring it in to Unity and then use it on an area that is 256x256, for example, just make the 256x256 to start off with).
One of my favourite tricks for images comes from Web Design and it involves using the lowest possible image size and cleverly repeating it for effect. See the “CSS Gradient Background” section of this link to see what I mean.
The advice above also applies to other things, such as models: are you using too many? Are they vital? Can you cut them down without losing the desired effect?
Always remember that you can reduce the weight of your game if you cleverly re-use prefabs, textures, sounds, models, etc.
Hope that helps!