Fuel hose connections on the mechanical fuel pump

Other making it really difficult to attach the hoses backward;
The Outlet hose is under pressure, but the inlet hose is under suction and subject to vapor formations (Vapor Lock).
A larger connector and hose reduce the partial vacuum and makes it easier for the Pump to get what it needs.

What Bill said.

And you want an ample supply of fuel available to make sure your system has enough  for all fuel demands.

Suction lines many times are larger than pressure lines. Technically the pressure side can move more fuel depending how things are setup and line sizes. (This can be a big factor in certain situations)

It also prevents you from connecting the pump backwards.