Detail this asset from Unity Store: Original Link
This is a paid asset, but now you can download Build Report Tool Free.
Build Report Tool v3.5.1
Build Report Tool v3.5
Trying to reduce your game build to less than 50 MB? Need to see which of your assets are eating up disk space?
This tool provides a nice front-end for Unity’s build info. It shows the assets included when you build and how much storage space each of them take.
Asset Usage/Dependencies: See which asset is using which, to let you understand why an asset is being included in the build.
Unused Assets List: Maybe you have files you don’t use anymore? See what assets are not included in your build with the Unused Assets List!
Save to XML: Save your build reports to XML files! Store multiple build reports for reference, or perhaps send them to your teammates.
Project Settings: See what build settings were used on the project upon time of building. Useful for Continuous Integration systems when reviewing settings on automated builds.
DLL List: See exactly what Mono DLLs are getting included in your build and how much space they take.
Also works on Personal Edition and mobile.