In the Android Publishing Settings you’ll find the option to split the application binary (.apk) into expansion files (.apk + .obb). To find it, go to Edit -> Project Settings -> Player. Then select the Android button, and expand the Publishing Settings section. The setting is a checkbox marked “Split Application Binary”.
This mechanism is only necessary when publishing to the Google Play Store, if the application is larger than 100 MB. See http://developer.android.com/guide/google/play/expansion-files.html for further information on APK Expansion Files.
When the Split Application Binary option is enabled the player executable and data will be split up, with a generated .apk (main application binary) consisting only of the executable (Java, Native) code (around 10MB), any and all script / plugin code, and the data for the first scene. Everything else (all additional scenes, resources, streaming assets …) will be serialized separately to a APK Expansion File (.obb).
The Split Application Binary option is not the only way to split an .apk into .apk/.obb (other options include 3rd party plugins/asset bundles/etc), but it’s the only automatic splitting mechanism officially supported.