Maximum weight of a pedal vehicle
According to Bikes At Work: 


  Our experience has been that most people can comfortably pull 300 lbs (137 kg) with a typical mountain bike and cargo trailer or cargo trike. How quickly a person can move load of that weight will depend on his or her physical condition. Someone in reasonable physical condition can generally pull a 300 lb load at 10 mph (16 km/hr) on level ground if there's no wind. A person exerting the same effort could pull a load of 600 lb. (275 kg) at a speed of about 8 mph (11-13 km/hr), and a 1000 lb load at about 6 mph.


At the page linked above there is a calculator:


  to estimate how much weight a typical healthy individual could transport on a warm day in still air using a bike and trailer or cargo trike. Things like wind speed, equipment condition, and personal health can have an enormous impact on results, so use this only for rough approximations.


Bikes At Work is talking about a bike pulling a cargo trailer but the information can be extended to other pedaled vehicles.

Once you get a weight moving on the flat you can move a lot of weight - given the right gearing given a reasonable level of fitness. Hills are a whole other world.