What people told you is correct. the only difference is the compressed version has reduced audio/visual quality to be installed on phones without enough space. I have tested both on my Pixel 3XL and I can't see a difference. You would only notice on something the size of a tablet or larger. A 5" phone screen simply isnt big enough to see the difference.
You need to have ~15GB of free space to install the full size apk. Android requires more space than the size of the APK to install. Thats a limitation of Android, not CoR.
Joiplay does work with cor, you need to have the renpy plugin and download the windows version. Follow the steps here: http://cor.fandom.com/installation