I am the developer. It was using the full video streams up until last week when I released version 1.8.
However it is possible that the android MediaPlayer when given a video stream, doesn't bother to download the video too, but I doubt so.
Now it specifically downloads audio-only streams, however only on Android 4.3+ devices.
Android 4.2 and lower still stream the full video because the OS isn't compatible with the audio only streams.
To give you an idea of bandwidth consumption, a 5 minute song takes up about...
- 3MB video stream, HD Audio disabled
- 12MB video stream, HD Audio enabled
- 3MB audio-only stream, if your device supports it. The sound quality is about equal (possibly a little better) than HD Audio with video.
On Android 4.3+ devices, the HD Audio switch in settings doesn't really do anything..