Airplanes are only so big. Given those dimensions the client (the airline) designs a seating arrangement to comfortably seat the most passengers possible. If an airline is able to fit 350 seats into an airplane, then that plane is allowed to seat 350 people. If there are 200 seats, then only 200 people can fit on it. I may be wrong in saying this (please correct me if I am) but the number of seats and related safety equipment (Oxygen masks, cargo space, lifejackets, etc) determines the maximum amount of passengers on a plane.

Total capacity is certainly limited by weight constraints. When airliners are designed, they are planned to accomodate their customers needs and the number of passengers they want to carry in markets where they will fly that aircraft. The designed structural weight limits on the airframe itself are the defining set of numbers that put a limit on what the airlines can do with an aircraft. (Airline marketing just can't put in 1,000 seats because they fit and then expect the plane to fly!) Beyond Operating Empty Weight/OEW there is only so much left over to accomodate passengers & cargo until you hit Maximum Allowable Take Off Weight/MTOW and that is the limit. OEW is the manufacturer's empty weight of the delivered aircraft plus the airlines additions such as: crew weight, crew baggage, cargo containers, oil, unusable fuel, food and beverages, galley equipment, passenger service equipment, emergency equipment, potable water, and entertainment systems. This does not include fuel, cargo, or anything that cannot be disposed of in flight. To summarize: includes everything but fuel, passengers, and cargo. (Of course max ramp weight is over MTOW by exactly the amount of fuel needed to burn on the taxi out, but that's a whole other story...) The speed at which the aircraft must be evacuated is another factor that can affect capacity. US Code of Federal Regulations; Tite l4 Federal Aviation Regulation Part 25 sets the standards that must be met for certification of transport aircraft. http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?c=ecfr&sid=624778693bda7a14a9dbc865854a2d25&rgn=div5&view=text&node=14:1.0.1.3.11&idno=14 Subsection 25.803-"Emergency evacuation" states: (c) "For airplanes having a seating capacity of more than 44 passengers, it must be shown that the maximum seating capacity, including the number of crewmembers required by the operating rules for which certification is requested, can be evacuated from the airplane to the ground under simulated emergency conditions within 90 seconds. Compliance with this requirement must be shown by actual demonstration using the test criteria outlined in appendix J of this part unless the Administrator finds that a combination of analysis and testing will provide data equivalent to that which would be obtained by actual demonstration." On another note, certain flights may be "weight restricted". What that means is that with a full load of passengers, cargo capacity at max allowable weights and full fuel, the aircraft is beyond it's MTOW. The only thing left to do to get the weight down for take off is to remove weight by taking off passengers, bags or fuel. Usually, a "weight restricted" situation arises when there is bad weather at the destination and the aircraft must carry extra or completely full fuel tanks in the event of a diversion from the destination airport. Hence, the weight of the extra fuel limits the amount of passengers & cargo that can be accomodated on that flight.