World Icons can't be larger than 64x64

I took a screenshot of the world from an angle that I thought would look good as the Icon for the world. I cropped it to a square and re-sized it to 512x512

What I expected to happen was:
The world icon would load normally at the size I set it to.

What actually happened was:
The world icon was the default icon.
Upon re-sizing the image to 64x64 it showed the image I took.

Tested with and without resource pack.