Why not? They played with as much or greater skill. Why should a few skilled players who try hard with the limited number of people and time they have not reap as good of rewards as a cluster fuck of n00bs who just go in and bash shit?
The thing about scaling is even as you make the fight harder and harder it generally still gets easier with more people just do to the ability to bring more specialized jobs or allow the jobs to play in a more specialized manner. Of course this assumes you take full advantage of the increased numbers and the players have some knowledge, ability and can work together.
Also if you could do interesting things with the treasure pool. Like have a combine one but make it so each extra person is another chance for something to drop. Or allow people to donate individual pools to the main one etc. While the amount of drops per person stays the same as if done with a smaller group you lower the chances of rots thereby increasing the quantity of drops useful to the people in the group.
Plus like you said some people just like getting together in a group