In light of the Guinea Pig infestation that has taken Orbis, which renders benefits by the means of referral links to mass recruited nations that will most likely go inactive eventually once they've hit the 500 Score requirement and deposited the bonus. After that is done, I'd imagine the over 1.2k nations with 10 cities would serve nicely as tax subjects, as well as a means by which to keep a slot in the leaderboard with bloated and not necessarily active low score nations. Should they get raided despite the low score, the main alliance is likely to counter with the active percentage of the sub-alliance, making it harder for raiders and estabilishing a hegemony in the low tier.
The way I see it, while the number of people in an alliance isn't really the problem here, though I believe a 300 member maximum is reasonable, GPWC in specific seems to be just the case I depicted from analysis, setting a precedent for other alliances to operate under the same premise, by which they can freely use this system to benefit themselves in a short amount of time. Such alliances should be monitored for various reasons, in order to avoid exploitation of in-game mechanics.