The icon on iPadOS and iOS is larger than the standard size.
That should be impossible. Agenda actually provides a standard square icon, and iPadOS “cut out” the rounded shape. So no app can make a bigger icon.
What is probably happening is an optical illusion. I often think some icons are bigger or smaller, but it is just because they are lighter or darker. It is an odd effect. You should find it is exactly the same size. I wouldn’t know how it could ever be bigger. It is simply not allowed by Apple.
Truth versus perception. Just measured the size and it’s exactly the same. Thank you for the clarification.