Why is the pump suction pipe often one size bigger than the pump connection?

It has become common practice to always have the pump suction line at least one size greater than the pump suction flange size. The transition is accomplished with an eccentric reducer usually with the flat on top but not always. The most critical thing about the pump suction is to ensure that the flow stream lines arrive at the suction eye of the pump with no large scale turbulence as can be caused by an upstream elbow. This is a function of the pipe geometry which means that a nice long straight suction pipe is desirable. A larger pipe reduces the pressure drop due to friction and provides more pressure at the pump eye, this provides more energy to the pump.

All kinds of cockamamie pump suctions have been designed in the past for various reasons and some of them even work. However, as a piping designer you don't want to re-invent the wheel all the time and you are looking for tried and true methods that will let you sleep easy.