Many RPG rules sets have a recommended or implicit target number for party size, as well as practical limits on what the game system can handle and keep things moving. When you read about the early days of D&D, it seems like those guys played with gigantic groups - 10 players or more! Those halcyon days haven't exactly returned, but my home group these days tends to be larger than the game groups earlier in my career - it now includes dads and some of their older kids, so our regular attendance is usually 7 players and a DM. It could easily be more if I had a bigger table and more chairs.
One wrinkle to consider as I compare Call of Cthulhu vs weird D&D horror is this party size factor. I feel like the target number for Cthulhu gaming is probably 3-4 players, though we've frequently had to stretch to 5. Whether the premise involves a pair of police detectives and consultants, a few Delta Green agents, or the traditional PI or jack-of-all-trades and friends taking on a job that leads to an occult investigation, the scenarios are intimate, and the nature of searching for lore and clues favors a small, focused group. It's comical to picture a flock of 15+ investigators (players plus retainers) descending on a small cottage in search of leads.
This reinforces my decision to stick with a D&D style system for any future campaigns that develop horror themes - whatever loss accrues in atmosphere due to the number of players, is outweighed because the setting and rules admit a larger play group at the table. We do have some explicit Call of Cthulhu plans brewing for in-between games, but we'll shave some group members for those scenarios to get to a smaller player set, keeping the kids out, for instance. I can think of a few Cthulhu campaigns that can support a gigantic party, too - something like Beyond the Mountains of Madness could probably do it, if we ever want to try an extended campaign using Chaosium.
Scaling D&D material to work with a large group wasn't much fun during the 4E era. I've got to remember to keep one eye on NEXT and see if WOTC is falling into the same trap of the "5-man party". If I see any marketing blather about the "party-size sweet spot" coming from those guys, I'm out.