big pump, little pump ?

[Oxymoron]

How come you can get more GPH out of a pump if you increase the pipe diameter? Does that still hold true if you say run 2 inch all the way then go down to 1 inch at the end?

Bringing your 2" line down to a 1" will reduce your entire flow to 1".

You would have to have the pipe and fittings 2" in order to not restrict your flow and essentially choke off your flow.