A problem I see with having smaller parties is that you then need a higher concentration of specific job types, namely mages and tanks. Given that gladiators are already fairly scarce at high ranks, and that 90% of DD players do not like to tank, I think that this could be quite a problem.

At the moment, with 15 person battles, you only need one tank for most things. If the max size was 8 you then need two tanks to have two groups run, and if you do not have enough then 7 people will miss out on the activity.

They should make an alliance two parties of 8, and have content difficulty scale based on number of people.