How do I properly make assets for mobile games

Hi, I'm fairly new with unity and that'll probably become obvious in a second.

I'm trying to make a mobile game but none of my custom button sprites look right. I made them in Illustrator using a Samsung Galaxy template so that the aspect ratio and everything was correct. Once I exported my buttons as PNGs and put them in unity they looked "okay" but when I ran the game on my phone everything was extremely pixelated. I'm not sure where I went wrong, they don't look pixelated in the engine preview and the game window.

Also, for clarity, to put my buttons into the game I replaced the sprite of existing buttons and tweaked the Rect Transform.

Edit: I've also noticed that if I build my app as an APK and install it on my phone, all of the buttons and text become extremely small.