How to export good quality 2D graphics for Android

shabby

Hi. I would like to know if there is any trick or any settings in unity that help keep the apk size less without compromising on the quality of graphics. I am making an android coloring and painting game for kids and it requires a lot of images but if I use them with size reduced they pixelate or look blurry. Any solution? I really think you should make a tutorial for this