Why are ski jumpers slim? Assuming perfect technique and material, how does the mass of a ski jumper affect the performance?
The higher potential energy of heavier jumpers could favour heavier jumpers, but then also friction and air drag would play a role. 
How do these factors depend on the mass and why are lighter jumpers favoured?
 A: I don't know if this is indeed so, but the trajectory is determined by gravity,  lift, drag, and initial velocity. Slim ski jumpers may have an advantage as air lift and drag are pretty much the same (they should mostly depend on the ski area) and the gravity force is smaller for them. Lift helps prolong the flight. There is a lot of similarity between a ski jumper and a glider.
A: Long distance ski jumpers benefit from maximising their surface area while simultaneously decreasing their weight. The less they weigh and the more drag they can produce, the farther they go. Their bodies are the primary source of weight and, as a result, there is incredible pressure for competing ski jumpers to be as thin as possible.
A: A less obvious reason is the effect of the "square-cube law" in biomechanics. Since the mass of a heavy ski-jumper will increase at a faster rate than the cross sectional area of the bones in his/her skeleton, a heavier athlete will be more liable to injury.
This is more than a theoretical consideration. The thigh bone of a "typical" human will break if a force of about 10 times the body weight is applied to it. The dynamic loads during activities like ski-jumping can be significantly bigger than the "$mg$" static loads caused by the same mass.
