My guess on 'hydraulic design'? As Carl said flow both thru the pipes and in the pool itself is important. I suspect the term is used to indicate that the pipes are sized and routed to minimize flow resistance. ex: swept bends vs. sharp 90's etc. The builder is right about 90's making it easier to fix a problem. However, by oversizing the pipes the bends and routing become less significant. The difference in cost between pipe sizes is small so once you dig, put in the largest reasonable size.
Al