Flow volume and velocity can only increase in a larger/wider passage if the pressure also increases. Flow volume can't increase in a fixed size passage (regardless of its size) unless the driving pressure also increases. Viscosity is constant for this discussion.Flow increases when the passages are wider with reduced pressure and pressure increases with reduced flow when they are smaller.
If you're looking at a PD oil pump running at a constant RPM, then the output flow volume is constant, and the flow pressure and velocity change according to the size (flow resistance) of the passage. Larger passage gives lower velocity and less pressure, and smaller passage gives higher velocity and higher pressure - but in both cases the flow volume is the same.
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