The problem is they as far as I know have no content which scales based on the amount of members involved. The coding for caravan is as simple as join event, get reward at end. There's no concept set in place to calculate involvement, check for afkers, or a means to scale it based on the amount involved.
Scaling an event is more then just adding mobs or making them stronger if 8 people fight 50 mobs, that doesn't mean 9 people fighting 60 mobs would be easier or tougher, it has to all be play tested to figure out the best setup for each increase in members.
If scaling events actually existed in this game then the empire invasion event would be a lot tougher then just pwning a bunch of lemmings for the slaughter lol.
The best way they can balance this event is by making it an instance and possibly adding it where you use a leve charge a run. Instead of risking leeches from joining your run it would be set in place like a primal run in which it will notify you that the leve/caravan is starting and everyone who posesses the proper leve will recieve a reward at the end of the caravan run.
Basically the chocobo dealer at the caravan could offer 4 different leves with difference challenges to them and then of course you have to be in a party when initiating the leve and upon starting it the leve/caravan is initiated for all.
Since the caravan itself would start automatically there would be absolutely no way for a leecher to involve themselves since if they attempted to enter a caravan solo then it would start off as a solo caravan run.
But ya, think that's the best way they could tweak it a bit, if they allow more people to do it they have to put a limit on it somehow and I just feel that using leve charges would be the best way to pull that off without making it to much of a hassle on the players.