Ah yes, good point.
You could take an average of all the pipe sizes connected and reduce flow to all connected devices equally but it would heavily simplify the piping mechanic, also it would be weird if you attach say a boiler and turbine to two sides of a huge pipe but then add a trail of mismatched pipes tacked on like a tail to another side of the pipe. It would need to calculate and reduce flow based on all those extra pipes.
I suppose you could calculate closest paths from generators to receivers and work out the flow based on average pressure + or - the pipe sizes in the chain but then things start getting complicated again, especially with modifying them on the fly.