ok..
i have just tested with bichon province and woomyon woods..
if you export bichon with a map editor you have a bmp which is 4200x2800 pixels
in the mmap.wil the big map for bichon province is instead 528x350 pixels
which means the bigmap is a 12,5% of the original
while the ww is originally 3600 x 2400 pixels
and it's big map is 452 x 300 pixels
which means the bigmap is 12,5% of the original..
i think it depends all on how big is the original map..
i would aim to get the bigmap around 500x325 pixels more or less..