They already said there will be raids like other games which will have more then 4-5 people.

Personally I would prefer small groups because anyone who has raided in wow or other games knows it only takes 2-3 terrible people in a 25 man raid to ruin it for the other 20+. With these smaller groups you can just form a pug and do these runs and if someone is bad or terrible it would be easy to see who it was and get a quick replacement instead of spending the better half of your night trying to fill your raid like any pug raid in wow. Guild raids are obviously different.

The other thing is the storytelling will be better with smaller groups getting everyone in on the dialogue cut scenes and each having their own part and choices. It appears they are catering to the big raid addicts from wow and the people who prefer the more personal experience with small mans and accessibility of small quick to form groups.