The Horde should retain control of the land they had before the War of Thorns. It’s in their best interest to maintain control of Southfury River. It’s Orgrimmar’s main water supply. It’d be foolish to be downriver of the Alliance. I wouldn’t expect the Night Elves to resort to poisoning it (although enchanting it in some way isn’t off the table), but it’d still be incredibly irresponsible for a nation to allow their enemies to control their water supply.
The Horde had 5* non-contested zones to the Alliance’s 6*. That alone is a ~17% difference. And of the contested zones, there were more zones that the Alliance could rightfully claim as their land despite Horde presence, like Ashenvale and Felwood, as well as a number of zones that were “contested” despite there being zero Horde presence, like Redridge Mountains, Duskwood, and Wetlands (Horde has no quests and no flight paths, they are basically Alliance zones that allowed PvP). The only zone I can think of where the opposite is true is Thousand Needles. And this is not even mentioning Theramore, of which the Horde had no equivalent version of (lol Stonard).
It makes sense story wise for the Alliance to have more considering the Horde was still very young, but it could also have made sense for that not to be true, say if Blizzard made different decisions in Vanilla. Night Elves could have been a Horde race, for example, and all of their holdings could have been Horde holdings. Or the Echo Isles could have been it’s own zone. Or the Barrens could have been split into multiple zones from the beginning. There are a number of things they could have done but they ran out of time because the Alliance side was developed first.
.* Durotar, Mulgore, Barrens, Tirisfal Glades, Silverpine Forest
** Elwynn Forest, Westfall, Duskwood, Teldrassil, Darkshore, Dun Morogh, Loch Modan
Edit- I misremembered Duskwood as non-contested when it was, in fact, contested, so I restructured the paragraph to reflect that change.