Many open-world fights did have a hard cap of 18 people in XI, though, with "spares" on the sideline in case something happened. But that's not really any different from Duty Finder automatically dropping in people if someone quits.
Dynamis Lord was an exception, and that fight required not just sufficient DPS (one of the few true DPS checks in XI) but a fair amount of coordination ahead of time. A successful DL back in the day was like a rocket launch. Everything had to be JUST SO to pull it off with anything short of 40 people. And the biggest difficulty with it was that you really only got one shot - fail, and that's it until the next Xarcabard.
Seriously though nothing beat the sweet sweet success of beating a hard Dynamis boss that decided to be generous!
Edited, Jun 9th 2014 12:31pm by Catwho