To make the problem worse:
- Ads are now bigger (300x250 instead of 220x250, 36% more pixels)
- High DPI (aka retina) means 4 times more pixels
As a result, even very simple ads miss the 40 KB criteria.
Given that some sites don't even mention High DPI yet, I am pretty sure that this 40 KB requirement is a leftover from ancient times. Could it please be updated? Or if it is genuinely a bandwidth concern, better state it clearly ("even for High DPI").
By the way, I suggest saying "High DPI" instead of "Retina" which is a brand. Written on High DPI Linux laptop. I also suggest implementing srcset or something similar to serve low-resolution images to low resolution displays.