This is not a problem with your configuration. It's a problem with how you have setup your asset. If your asset is 45pt, UIButton will not render it bigger than 45pt even if your button is 300pt. If you want the icon to truly scale to any size, as you already discovered, you will need to use single scale. Single scale means that you are providing a vector asset so system knows it can scale the image to any size without distortion.

TLDR: Stop using pngs and go with an svg or PDF and use Single Scale.