It's all about the details
Stair runners can be tricky, so getting it right is extremely important. A runner made too narrow can look skinny and out of balance relative to the width of the stairs, too wide and you run the risk of loosing the 'accent' of the uncovered element at the sides. The other consideration is how the curve on staircases with triangular/kite shaped stairs is to be negotiated. Here's an example of a project recently completed, note the finishing at the base of the runner in the first step of the main staircase and the transition to and from the triangular stairs.