Android 8 Oreo brings some changes regarding the Add to Home screen feature. One of them is that it makes your icon fit a circle no matter what.
Android expects your icons to fit in a circle. For example, these icons:
Become:
What happens if the icon is a little “more” than a circle? There is a slight tolerance. When editing a 192×192 icon, the circle can be extended up to 2 pixels:
Beyond this, Android resizes the icon to make it fit:
Oh, and there are apparently no low limit. As long as your image fits in a circle, Android does not edit it:
