I thought the admin could change the different size categories higher or lower and then after some later changes there might be different values for the uploaded medium avatar size and the style medium avatar size. If this is not the case it is not a bug.
I use a single Scalar Vector Graphics for all my guest users, and this image fits perfectly in every location (small, medium, big, even the profile page as I uploaded the image with this size). Just at this particular popup the vector doesn't scale.