Small applications that download quickly generally give the best user experiences. However, if your application requires more than 50MB of space, you can use expansion files to store additional APK assets. We will store two files (in addition to the original 50MB) of data per application at no additional cost. The maximum file size for each of these two files is 2GB, but it’s better to make those files as small as possible for the best user experience.
Google Play hosts these files for your application. When possible, Google Play will download these files whenever it installs or updates your app. In some cases your app must still download its expansion files, but Google Play always provides the hosting, and code is provided for obtaining the URL and other download information.
The expansion files are saved to the device's shared storage location (such as the SD card or USB-mountable partition) where your application can access them.
A single APK can be supplemented by two files, each up to 2GB in size. One file is the "main" expansion file and the other is an optional "patch" expansion file, usually used to provide small patch updates to the main expansion file. Every extension file URL issued is unique for every download. The expansion files can be any type of file.