Just a little water flow dynamics for you, there is a standard equation we use to calculate water flow which is:

PV = nRT

Where P = Pressure V = Volume, n = number of moles R = universal constant T = Temperature. Usually the nRT side is constant so in a pipe we look at:

P1V1 = P2V2

So in a case with a pool where a pump is pumping at a constant volume rate the pressure would change. So here if a pipe diameter decreases the pressure would have to increase. Depending on the size difference you can calculate what the exact pressure change will be.

Eric