They all ready told us that when the Borg assimilate a civilzation, they assimilate their resources as well.The Borg like the Federation travel space looking for other species to "join" them. It would stand to reason that they'd travel space more after the war to assimilate more species to make up for the numbers they lost. Just like I'm sure after the Dominion war, Starfleet is looking for new recuits to make up their lost numbers. Why build space fairing ships if you aren't going to fly thru space in them?
People of all races on Earth don't stay within the countries of their origin. If so, places like America & New Zealand wouldn't be colonized.
Starfleet & the Borg are equally the same in their goals, to seek out new life and new civilizations. They are both the same, just opposite sides of the same coin.
Oh, I've not questionning their INTENT to do what you're saying. What I'm questionning is their AVAILABLE RESOURCES after being in a devastating war. There would normally be some years of rebuilding to get back to where they were before they could start expanding past the point they were at before the war.
Remember in "BOBW" the entire city was gone?