The trick to working with larger groups of players is to give them something to do, preferably something that they can do without much input from you.
I typically only run CoC at conventions; the only time I get to run it at home is when I playtest for my con games. One of my best experiences as Keeper was 2 years ago at Origins. After presenting the initial scene and calling for a few dice rolls, I barely spoke for the next hour. Characters were arguing with each other, trying to come up with plans, etc. I mostly just answered questions like "Are the police here yet?" or "Is there somewhere in the basement we can hide the girl's head?" You know. The usual.
Part of it is going to depend on the players. Some players will jump right in and run with it, others will need a little more coaxing. One of my worst experiences as a GM was 2 years ago at Gencon, my last slot of the con. I only had 3 players for a Buffy event and they were all of the "need a little coaxing" variety. (And I was exhausted and hungover. That didn't help.)
As a player, I played in a CoC game at Gencon about five years ago that was slotted for 8 and ended up with about 12 people. 11 of the 12 people had a blast, in part because we had plenty of things to do and discuss among our sub-groups.
I guess my point (at last!) is that your mileage may vary. Some game/player permutations will work great with a large number of players, some will not. Be prepared to work for it.