We have Word of Devs that the sectors are not created at game start but are generated on-the-fly.I am just guessing here but I believe that sectors are created at game start and assuming that P-Dox made them this way for performance issues. The only thing I don't like about the new system is I can not remove "finished" planets from my core list. Previously I could just dump a few planets in a sector to both clean up my core list and forget about them.
Anyway, since I'm not going to be playing Ironman until at least the population growth issues get worked out, I've got no qualms in my single-player games about editing the save file to adjust sector boundaries.