I'm not completely sure of your question, but pump sizing is related to static head pressure (water height, to over-simplify, in this case zero) as well as dynamic pressure from friction in the piping. There are formulae and tables for calculating equivalent head pressure based on pipe size, fittings, and (one of the most important) water flow velocity. Dynamic head pressure (or more properly "frictional head losses") vary as the square of the velocity, so larger pipe makes a huge difference.
Placement of the pump in a closed system makes no difference. But an overflow may not be a closed system in a pressure sense. Water falling freely into a sump for example "wastes" the energy as it falls. Pumping back up from a sump into the main tank then has the static head pressure of the difference in water levels in the main tank and sump, PLUS the dynamic head loss from friction due to length and size of pipes.
So your comment on overflow seems at odds with the comment on it being a closed system. Unless the overflow is kept completely full at all times, somehow.