That's interesting, can you expand on this? Is it because more players will have more interplay with each other, and thus require less content during the session? Likewise, you'd need to do more one-on-one personal engagement with each PC, whereas with 6-7 you can focus on the story arc?
It's partially that, but only partially, and almost its opposite. I'm struggling to explain it in a couple of brief sentences, so bear with me...
My platonic ideal of D&D is an
aggressively character-driven sandbox. I don't plan a campaign beyond the "elevator pitch" I offer my players and sometimes the first adventure. What I do is set the tone, come up with a handful of
elaborate plot hooks, and then everything that happens in my "campaign" is the ongoing consequences of the players' choices in regard to those initial plot hooks. People they injure or insult nurse grudges and plot revenge, the delicate checks and balances they disrupt realign themselves, power vacuums get filled and power players react to new competition. So having more players isn't a matter of everyone "focusing on the story"... it's a matter of more players providing the "one on one personal engagement" for each other.
It actually ties into my "no backstories" houserule/philosophy: it seems like it contradicts "character-driven" roleplaying, but the idea is that if all of a character's "background substance" fits on an index card, well, all of the other players can memorize six or eight index cards' worth of background substance and engage with it, compared to three or four times three or four pages of background substance that they'll never read.
So with more players:
- Collaborative setting design, which I love for settings like Spelljammer or my own Shroompunk, works so much better with more players.
- More inspiration for plot hooks, and more wholecloth plotlines, that I don't have to come up with myself. Any one PC's "personal storyline" can suddenly become the "main plot" at any time, and nobody complains I'm playing favorites because it was everyone's choice.
- More PC-to-PC interaction and player engagement with eachother's BS means that their "personal storylines" become interconnected organically, multiplying my "one on one engagement" by two or more characters and making it seem like part of the campaign.
- Larger combats in any edition of D&D take more time to resolve, but multiple player interactions during combat keep them from feeling as grindy-- especially because larger parties can both punch well above their weight class, and survive defeats that would wipe a smaller party.
- The obvious common threads between these points are "getting players to do my prep work for me" and "keeping players occupied with each other while I plan my next move". Less obvious, but more critical, it relies on the players keeping each other engaged and actively putting each other in the spotlight.
Basically, my entire playstyle is based on the idea that six players can do the DM's job better than one DM can, but no matter how much I congratulate my players for how clever they were... I still end up getting all the credit.