In my experience, mobs have always presented the bigger threat to PCs than big-bad monsters. There are a number of reasons for this, some mechanical, some psychological. Why does this seem to be the case?
First of all, adventurers are usually prepared for a monster. They have often heard of the monster they will soon fight and have taken precautions based on the information they have gathered. Indeed, the reason they might be in a specific location is for the sole purpose of finding and dispatching said monster.
When fighting the monster, there’s often an economy of force which the adventurers are able to match 1/1 or better, whether it’s in terms of damage, overall hit points, or most significantly, perhaps, number of attacks.
One large monster will typically get to make 1 attack for every 3-6 attacks it receives; even if it is doing more damage and hitting more often, PC tactics can often compensate for hits and spread damage in an effective manner to minimize irrecoverable losses.
Mobs are different story. Even if the players are prepared for a big fight, they may not be prepared for handful of mooks that are waiting at the mouth of the dungeon to take the treasure the heroes just recovered.
The PCs’ economy of force may be matched or reduced. Mobs will often be attacking at a 1/1 ratio or better; the man-to-man fighting will also prevent use of certain tactics which the PCs might more effectively use against stronger foes who are fewer in number.
Oftentimes, the most devastating party losses come at the hands of a mob AFTER defeating a large monster. Why? Players assume an air of invulnerability after successfully dispatching single dreadful foe, but are brought low in an evenly matched fight when forced to fight one-on-one with few or no assists from fellows.
Does this jibe with the ‘heroic’ notion so woven into D&D?
I think it does.
Many iconic heroic battles throughout history and literature consist of 1-v-1 fights or one or a few heroically holding off a much larger force until they are wiped out.
On one hand you have Beowulf & Grendel or David & Goliath, while on the other, you have Benkei at the bridge or the Spartans at Thermopylae. One advantage of a game like D&D is that the game isn’t over for the player when the guy or guys left to cover the others’ retreat finally succumbs to the tides of battle. They can just roll up a new character. Of course, this doesn’t work if you’re building a fictional character franchise – Conan can’t be killed by mooks (though he and many other pulp characters have come close to being brought down by them many times). But I don’t think that characters dying to mobs is necessarily antithetical to pulp-style heroics, since those heroics draw heavily on earlier literary heroic traditions, ones where heroes DO die.
And when a character makes a heroic last stand, that character is gonna be remembered.
Now, there ARE mechanics that do give PCs an advantage over mooks in ways that reflect those scenes of one character killing dozens. Fighters get extra attacks against single hit-die opponents. 1HD monsters and most human opponents should fall into this category. Mid-level fighters have a decent chance of cutting through several such opponents each round! Can they get overwhelmed? Absolutely! Lots of mobs are going to be tougher than 1 HD, but then that’s not like a hero being overwhelmed by mooks, is it? That’s more being overwhelmed by not one but several monsters.
Sometimes, heroes just need to run away. Plenty of pulp S&S stories start with the hero running from a fight that they know they can’t win, usually involving a large number of mooks who are after them. The difference between your characters who died and Conan could be that Conan knew when to run and you didn’t.