There is somewhere else, in this forum, a discussion like this.
http://forum.m0n0.ch/index.php/topic,4811.0.html#msg14822It look like that m0n0wall doesn't has a dinamic "adjustment" of the bandwith according to the size of the download.
There is a traffic shaper which adjust the limit of the bandwith according to the "position" in the networks of the m0n0wall You're using. I mean, each IP has his bandwith allocation.
I believe that someone has request to the admin to add this feature in the future.