The conical manifold is beneficial due to the fact that the exhaust gasses "stick" to the walls of the pipes it is travelling in. This stuck layer does not move the same as the remainder of the gas, thus it effectively reduces the pipe size by the thickness of this layer. The layer thickness depends on several factors, density, temp and other reasons.

A conical shape to the pipe massively reduces the thickness of the gasses that want to stick to the pipe walls, effectively increasing the usable internal diameter of the pipe.