I think there's a direct relationship between the ease of a TPK and the number of PCs you have at your table.
The smaller the number of PCs, the easier it is to TPK them. And this is for a couple reasons. First, the smaller the number of PCs, the less likely they are to have enough healing support to keep themselves up and active. If you only have 4 PCs, odds are much better you only have a single one that can heal. Which means that if someone hits 0 HP, it's harder to get them awake again. Thus they have better chances to fail Death saves, and/or better chances to have allies far enough away that monsters are available to hit them when they are down.
The more PCs on the field, not only is that more HP you as the DM have to grind down... but there's many more people available to keep the ones who get knocked out from actually dying-- even if they don't necessarily heal them, they at least can stabilize them so they won't die. But most likely they WILL heal them enough to get them back on their feet and back into the fight and thus you're back to large numbers of available PCs to run to each other's aid.
But I think there's also a psychological issue DMs have when they have tables of large number of PCs: Because of the fear of making long, drawn-out, grindfest fights, they are less likely to really throw enough enemies on the battlefield to make a challenge for tables of large numbers of PCs. A table of 8 PCs means those players are waiting a LONG time between turns-- especially if they aren't so up on the rules and tactics that they know exactly what they are to do and can run their turn immediately. Now a DM needs to put enough monsters down to challenge that table of 8 PCs? Forget it... that would make most fights interminable. 8 PCs and like 15 to 20 enemies? No one wants to run a fight like that. But when you have things like PC power, focus fire and the like, you NEED that many enemies to even give them a shot at a TPK. Because you need that many to occupy all the PCs so they are less likely to be able to get other PCs back on their feet, and thus move your way towards a TPK.
And on top of that... there are not many DMs who can effectively plan and run a 16-20 monster squad with effective and strong tactics, while at the same time trying to maintain table interest and cohesion and story. And thus any fight against a table of 8 PCs is going to be less tactically effective than against a small group, which is the exact opposite of what you need in order to take down a group of 8 PCs.
I'm a narrative and story-focused DM so I don't really have any desire or care about trying to TPK my players-- I use other things to generate story tension. But the deadliest and scariest battle I've ever run was for 3 PCs against a single banshee. That's it. That's all that was on the table and it was scary and deadly mainly because it played SO quickly that all 3 players maintained engagement, only a single PC had any sort of healing capability (which meant that should one of them have dropped to 0 HP odds were good a party death spiral was going to happen) and I was able to run that single monster much more effectively tactically since that was all I was focused on.
You don't get that kind of action usually when you're running a game for 7 players. It just doesn't happen. At least not with any regularity.
The smaller the number of PCs, the easier it is to TPK them. And this is for a couple reasons. First, the smaller the number of PCs, the less likely they are to have enough healing support to keep themselves up and active. If you only have 4 PCs, odds are much better you only have a single one that can heal. Which means that if someone hits 0 HP, it's harder to get them awake again. Thus they have better chances to fail Death saves, and/or better chances to have allies far enough away that monsters are available to hit them when they are down.
The more PCs on the field, not only is that more HP you as the DM have to grind down... but there's many more people available to keep the ones who get knocked out from actually dying-- even if they don't necessarily heal them, they at least can stabilize them so they won't die. But most likely they WILL heal them enough to get them back on their feet and back into the fight and thus you're back to large numbers of available PCs to run to each other's aid.
But I think there's also a psychological issue DMs have when they have tables of large number of PCs: Because of the fear of making long, drawn-out, grindfest fights, they are less likely to really throw enough enemies on the battlefield to make a challenge for tables of large numbers of PCs. A table of 8 PCs means those players are waiting a LONG time between turns-- especially if they aren't so up on the rules and tactics that they know exactly what they are to do and can run their turn immediately. Now a DM needs to put enough monsters down to challenge that table of 8 PCs? Forget it... that would make most fights interminable. 8 PCs and like 15 to 20 enemies? No one wants to run a fight like that. But when you have things like PC power, focus fire and the like, you NEED that many enemies to even give them a shot at a TPK. Because you need that many to occupy all the PCs so they are less likely to be able to get other PCs back on their feet, and thus move your way towards a TPK.
And on top of that... there are not many DMs who can effectively plan and run a 16-20 monster squad with effective and strong tactics, while at the same time trying to maintain table interest and cohesion and story. And thus any fight against a table of 8 PCs is going to be less tactically effective than against a small group, which is the exact opposite of what you need in order to take down a group of 8 PCs.
I'm a narrative and story-focused DM so I don't really have any desire or care about trying to TPK my players-- I use other things to generate story tension. But the deadliest and scariest battle I've ever run was for 3 PCs against a single banshee. That's it. That's all that was on the table and it was scary and deadly mainly because it played SO quickly that all 3 players maintained engagement, only a single PC had any sort of healing capability (which meant that should one of them have dropped to 0 HP odds were good a party death spiral was going to happen) and I was able to run that single monster much more effectively tactically since that was all I was focused on.
You don't get that kind of action usually when you're running a game for 7 players. It just doesn't happen. At least not with any regularity.
