I'm finding that ETC1 textures (.pkm) are taking a rather large amount of data up on my storage device. What options are there for further reducing the file size, such as compression with a third party like zlib, anyone know how to do that? Is there any other techniques to reduce the file size without compromising quality?
It is worth pointing out that this latter method of separating the 'assets' from the apk is a very common thing done with many games that are large in size. Originally because of the limitation of APK size for the Google Play Store, but is now a standard practise amongst all games that require large amounts of assets.
Google even allow you to separate them and store them on their servers, these files are usually referred to as 'obb' files. I believe you get 2 as a maximum, each being no larger than 2GB in size. If your game needs more however, you will need to have your own server infrastructure in place that your game can connect to and download.
A further benefit of having your own server, is to allow you to separate the assets by device and/or performance of the device. For example regarding textures, you can therefore have ETC1 texture pack, as well as an ASTC texture pack, and upon first launching the game, it can detect if ASTC is available, and if so, download that pack, falling back to ETC1 if that isn't the case. This means you don't require everyone to download all of your different compressed textures, when most will not be used for their device, further reducing the overall size of the fully installed game.
For further information on these techniques, please google e.g: APK Expansion Files | Android Developers
I hope you find this useful.
Kind Regards,
Michael McGeagh