Unity 2D build for android, memory issue. Sprite import?

Hi, I searched a lot about this topic, but I couldn't find a final answer for my question. I'm trying to build a simple 2D game on android, so I'm using many PNG files as 2D sprites (with alpha channel). Everything was fine, until one day I installed the apk of the game (which worked fine in the editor) on my smartphone and it quitted itself during a scene loading. I figured out it was a memory issue and, since I was just dropping my PNG into unity, I started to dive into the import settings, but I couldn't find a specific universal way to import my PNG in order to use the essential memory. Which optimizations should I apply? Is something I have to do after or before importing them? How is possile that some PNG of just a few KB could stop my game from running? Thanks for any advice you could give me.