We created a small guide on IL2CPP build sizes. What is important, what can be improved and how you can improve it. We constantly work on the improvements and could already gain massive size improvements in the latest patches. Ensure always to use the latest patch, as we fix bug on daily basis and work on size improvements in parallel. Image may be NSFW.
Clik here to view.
Although, many parts are similar to WebGL this guide is mainly targeted to iOS. But all the fixes we make for IL2CPP have direct affect on WebGL and iOS...
IL2CPP build size improvements
Clik here to view.

IL2CPP build size improvements