While the idea that a rapid transition to a larger cross-section doesn't hurt flow seems logical, it turns out that the exact opposite is actually true. A large portion of what I do for a living involves fluid flow analysis and what I have found is that an abrupt enlargement can actually be more harmful than an abrupt contraction in cross-section due to the higher velocity and the turbulance and flow separation that occur at the transition. What this means is that you will want a smooth, long transition with any change in cross-section area.