So I know this title sounds weird , but hear me out!
So... I have set the icon for my app. It was a really big (like 128x128) icon. But when I release it and install it it only has a small icon.
https://i.stack.imgur.com/Diy7x.jpg - my ico
But I want it to be big like
https://i.stack.imgur.com/NwhQS.jpg - spotify ico
Please help!
Windows is likely ignoring your pretty 128px icon and falling back to an upscaled lower-quality one.
I've followed the advice of this question in the past, and it's resolved this kind of issue for me. Essentially, the 48px and 256px images are the most important components.
You could also try reverse-engineering the header of your Spotify icon to see how they do it. The ICO format is well documented on Wikipedia.
