Every map to date (well over 100) would have to be changed if we changed the font size, because all maps were created based on the current font.
I agree that a larger font would make the numbers easier to read, but it would also take up more space and space is at a premium on most maps so that does not seem like a good option, even if someone was willing to change about 300 current maps (twice as many because of the large and small versions)