With the way Graal Online works, this would not save much space for people at all. Assets are downloaded as you need them(IE someone has a hat you haven't been near before, it downloads it, or a new level/sound). The actual raw size of the app (<20MB) is nothing compared to how big it gets after you've downloaded a lot of content.
If your phone is low on space, try reinstalling one of the Graal apps. This may cause you to lag a little at first sice you need to redownload images as you see them(thus increasing data usage too if you are limited), but it clears a lot of stuff you have saved.
For example, Graal Classic takes like 20 MB upon the initial download. As I continued playing, stuff gets downloaded and now it uses over 500 MB.