Just an update on this, I've determined that I am actually using 130x130 px images, as using native 13x13 looks quite terrible on its own.

I've exhausted every option I can come up with for scaling down circles via border-image (any circle scaled down to less than 30 or so px has this issue for me on Windows) that don't look terrible, and have simply given up and used a 13x13 image for Windows builds only (which look bad in comparison). I'm game for any advice on how to get the scaling to work properly, but otherwise this hack seems to be the only functional solution.