Yeah, we don't know the exact numbers involved, but we've known for quite a while that the reason no side has been able to take over the world has been one of economics (making Moneymaners the most powerful casters?). As your number of cities grows, each city gives a smaller % of its full income, eventually possibly resulting in a loss in income by capturing a new city, or an increase so small, that it is negligible. At the same time though, you still need units to defend your cities. That leaves you in a situation like Haffaton, in which most of their cities are virtually undefended, because they cannot afford to keep a standing force there. They would be a prime example of a large side that has shifted all of its troops to the outer cities, and left the inner ones with only token forces. They back this up with the poisoned provisions, but it is still tenuous.
Based on Olive's statement in this update, they only have a few hundred actual troops, the rest being uncroaked or garrison units (most likely those pea shooter plants). Compare this to the few thousand that Jetstone had. Even Jillian with her tiny side found that to be a small number.
As others have said though, Wanda can break this with Decrypted. They have no upkeep, and so schmuckers aren't really a concern for her beyond her own upkeep (in a theoretical side that she alone controlled). This would mean that she could theoretically capture every city -and- leave it with a good sized standing force to prevent counterattacks and popped barbarians. Something in the rules of Erfworld may make the barbarians pop strong enough to take a nearby city or something, but if not, then barbarians would be unlikely to do serious damage, and Wanda could use some Decrypted Dwagons to be just about anywhere in Erf on any given turn, meaning she could replace the losses with the barbarians that were killed. In fact, the longer she held all of Erf, the stronger she would get, because she could (if she really wanted) pop units in every single city she owned, then use a Decrypted Dwagon relay to visit every single city, kill all the popped units, Decrypt them, and continue on. So basically every city in Erf would contain the strength of its full production capacity. Like I said, Erf may fight back with strong barbarians, but they'd have to be amazingly strong to be able to hold out for more than a few turns against the full production capacity of Erfworld. And all that is assuming she could capture every city in Erf first and set up things like the Dwagon relay of course, which is more than a tiny challenge. So, there would be peace on Erf, but everyone would be (un)dead.