From Just Solve the File Format Problem
XAPK (APK Expansion Files) is an ancillary file to the APK file format. Where Google Play requires the compressed APK file to be under 100MB.
According to the Android Developer documents an XAPK file can be used for:
- high-fidelity graphics,
- media files,
- or other large assets.
An XAPK file is a ZIP archive like APK and may have a similar layout, as well as the additional assets, which may also be zipped and optionally encrypted, see OBB.