Fluid flow is significantly different to electricity in practice despite the claims otherwise. Flow potential increases with a radius^2 relationship, and the conservation of mass/volume has to be maintained.
So essentially if you stick to the simple A1.V1=A2.V2=volumetric flow rate. A and V being Area and Velocity then you can't go wrong.
consider if you push 300rwkw through a 75mm (4415mm^2) pipe, and you increase to a 100mm pipe (7850mm^2) and the gas velocity remains the same the potential flow for power is around 530rwkw (which is far more than linear relationship attributed to just the diameter increase)
Moral, 100mm pipe is just too big when 75mm (3") can deliver well over 300rwkw.