Bob,

Doubling the diameter of the pipe increases the cross sectional area by a factor of 4. It increases the flow volume at a given head (differential pressure or pressure drop) by a factor of 16.

Wire is different. Resistance is directly proportional to length and inversely proportional to cross sectional area (neglecting skin effect). You must choose how much resistance you are willing to tolerate between your amp and speakers and this will determine the wire size you need to use.