The Surly Big Dummy is rated for 400 lbs.
The trailer is rated for less, but that doesn't include the rider's weight (like the Big Dummy does).
On my bike and with that trailer, I could haul 300 lbs of stuff. On a Big Dummy I could only haul 200 lbs of stuff, because I would have to include my body weight in the total.
My current trailer is rated much lower (100lbs) mainly because it has a plastic panel floor with only one metal crossmember (at the the axle). I could always beef it up if I wanted, but honestly I don't see myself going over the 100 lb. limit.
"If Americans want to live the American Dream, they should go to Denmark." - Richard Wilkinson