I think SG's thinking on this is that if there are 18 members, and half are inactive, at least thats still 9 ppl giving good ideas rather than half of 12 (6). I realize that ideally you'd have the 12 best as the chosen, but realistically great people can end up going inactive for reasons outside the game, so having a larger number in the council gives a better chance of having a higher real number (not necessarily percentage of council wise) of active people in it contributing ideas and helping.
This is exactly the reason.
With current Council, after the initial excitement, I would say that roughly half of Council Members stayed active, maybe slightly above in some topics, but even lower than that with others. When I went to see the list of Council Members the other day, I was actually surprised to see one name there, because I couldn't remember this person posting once on the Council section. I didn't even know that this member was a Council Member. One other I knew was a Council Member, but again, I didn't remember one single post that this person had made.
So to those who were concerned that the Council section would somehow be overwhelmed with ideas and posts, I doubt that it will happen. 12 was not enough. Having that extra 6 might help. Sure there's always the possibility of all 18 becoming highly active, but I don't see it as a huge problem. We have a limit in active Council topics to prevent total chaos.