I can offer some insight here.
A fully loaded Q400 can not depart from CYTZ.
Many things go into determining how much weight you can safely get off a runway. The manufacturer has graphs and tables of performance, which you compare to real world conditions. The warmer it is outside, the less performance any aircraft is going to get. Barometric pressure, wind, runway surface condition, aircraft OEW (aircraft of the same type weigh different amounts), departure procedures, TORA, TODA, etc. It all factors into the RTOW. On a short runway, the RTOW is always lower then the MTOW, sometimes differing by 1000s of lbs.
On hot days in Summer, Porter (the airline you speak of) blocks off seats, to ensure the Q400 can meet the RTOW for the runways in CYTZ. I think Sami's interpretation is good at the moment, it presents a realistic operational picture for operating out of Billy Bishop.
edit: Forgot to mention fuel. A short trip with a good alternate, or no alternate, requires less go-go juice in the wings, so you can get more payload on an aircraft. A longer flight is going to eat into your Ramp weight and consequently limit your payload on a short runway departure, especially on with stupid Noise Abatement like CYTZ.
The Community around Billy Bishop needs to go on a long walk off a short pier... I might be biased here.