Plumbing sizes should be based on the size of the pump, what you are using the line for and also meeting minumum codes for velocity. There is no one size fits all.

Its the returns into the pool. 1.5 hp Pentair challenger with Sta Rite system 3 sand filter. Suction lines are all 2 inch with Jandy valves to adjust at the pad. I wanted 2 inch on all plumbing as I wanted to be able to move more water then I could previously. From the limited information I could find online seems there should be some improvement from what it was before even if its "choked" down to 1.5 inch plumbing for the last 12-14 inches of each return into the pool. I found several "white" goods manufacturers including Pentair and Waterway that have 2 inch wall fittings but seems they are all designed for 1.5 inch return fittings anyway.

Yep 1.5 at the returns is standard...

Common and perfectly ok