no i'm not really sure why some are thick and some are thin? the y-scale doesn't change in an equirectangular map, only the x scale. borders are supposed to be infinitely thin, we just use two pixels so we can see them accurately, but they won't be affected by the scaling.
Well if you put them onto a sphere they will be stretched closer to the equator and become thinner near the poles. (Ignore about my drawing which does the opposite, if they're constantly two pixels thick they will appear thicker near the equator).
But yes, I agree with Atomic. We could try to wrap it around a sphere and see if anything looks odd, but I think setting up rules for it makes it unnecessarily complex and something I wouldn't want to have to think about and calculate when drawing a map.