Going by this beta those numbers can relate to there being 4 zones.
Story missions, PvP maps, strikes and exploration areas seem to happen in the same maps as detailed in my previous post. Only thing that unlock while doing the missions/strikes are the boss battles. IE: The only difference between the earth strike mission, and you exploring on your own is the access to the very last boss and the two guaranteed items you get as a reward for finishing it (never seen an actual drop from a boss).
If there are 4 zones, we have close to explored 50% of the available zones at launch, minus the areas tied to the bosses/missions which tend to be one room/walled off wing, of the normal accessible areas.
Might be wrong, and it is larger, but unless the later areas drastically change how the missions represent the areas they are in, my guess we are missing Mars + one other planet. If this is the case, it is, as pointed out by others, much smaller than Borderlands 2.
Granted this is first person and The Devision is third person, but they are both next gen co-op explorationish games, and currently I think The Devision seems to have the edge, mainly because my hype bubble burst by trying this beta. Good game, but that's about it. Not going to cancel my pre-order and will get the season pass, but doubt it will hold my attention for long bursts at a time. I see myself finishing the content, not caring at all about grinding the available content 200 times to get "light" levels, and coming back to try new content. With the match making system and the lack of chatting, there will never ever be a case of needing to gear up to get to join raids. You will unlock the requirements, what ever they are, click launch, and get selected into a random group of people where the only interaction will be to press square to revive. Gear progression will be purely for your own merits.