Reducing size of executable?

After submitting my game to the app store I got an e-mail from apple that my game exceeded the cellular download limit by 7 MB. As is (7MB over), my game will have to be downloaded over wifi.

Suggestion for how to reduce size? I believe all images are resized by Unity when I load them in. If not, can I adjust them to reduce size in Unity?

Can I change the game to not use Unity code that is not needed? Am I able to clean out unneeded things in standard assists folders? Not sure how much this would save....