It's not necessarily the duration. It has more to do with when the intake valve closes. Imagine just one cylinder. When the piston is at the bottom of the intake stroke just starting it's way up on the compression stroke. The sooner the intake valve closes, the more mixture is trapped in the cylinder and the more pressure will be created at TDC. If a camshaft has a later closing event, pressure starts to be built later and not as much pressure will be produced. Is that simple enough?