Sudden changes disrupt flow.....Everything wants to be gradual change to maintain the highest flow and to keep from introducing turbulence. The exhaust coming off the turbo wheel is VERY turbulent and you want to straighten and guide the flow as simple and easy as possible. If you take the turbulent flow coming from the exhaust wheel and go to a much larger area the direction of flow is not reached and the exhaust whips around and slows and builds back pressure (Literally bouncing around) . Making a gentle increase in size after the turbo creates a vacuum sucking the air out of the turbo and also creates a direction for the exhaust to go increasing the speed of the turbo. The other issue with internal wastegate turbos that do not have any division and share the same housing opening is the fact the wastegate dumps exhaust across the turbo wheel outlet making crazy turbulence and slowing the flow.
