How does the automatic image resizing in reply posts work?

I can't figure out any rhyme or reason to it. It doesn't seem to go by file size or image dimensions. In this post I made, the exact same image that goes to the exact same link is being resized multiple different ways in the same post. They are also being resized differently from previous posts in the same thread using the same images or images of the same size. Sometimes, larger images will not be resized whereas smaller images will be.

I could handle it if the image resizing were at least consistent, but it seems to be entirely randomly applied. Ideally we could get rid of the resizing altogether (OP posts do not have the same kind of automatic image resizing), or just implement it for images greater than a certain size.