Drew, in case Rex doesn't jump back on soon, I'll try to explain what undersized pipe does to a pump...Ultimately the pump will have to work harder to get the air to the pond with smaller tubing. Try to blow through a coffee stir straw, then a soda straw. The small straw is much harder to blow through. The pump might be able to do it, but it can do it easier (more efficiently) with larger tubing. If the pipe is actually too small to carry the CFM you want then the relief valve pops open and the air would be released in the pump house and never make it to the pond. Some of the air would, but not the excess. The pump is saved, but the air is wasted.
Since you're a mathematician maybe you dabbled in physics a long time ago...
...you might recall that resistance to laminar incompressible fluid flow through a tube is inversely proportional to the radius to the fourth power. Air is compressible and there's a whole field of engineering dedicated to computing its flow, but the relationship remains exponential.
