Christopher Rice talked about how his (and other) pure beer & pretzels Dungeon Fantasy games start to inevitably morph into larger, deeper campaigns.
But this doesn't have to be.
If you basically all agree on a Beer & Pretzels game, here is how you keep it from slowly but surely morphing into anything bigger. I think most readers and all of my actual players will agree that my Felltower campaign has been straight-up old-school inspired DF since session 1 and all the way through to our latest, session 65.
Choose Shallowness over Depth
This might seem like I'm being sarcastic, but I'm not. Inevitably, as you play in a series of connected sessions, you start answering questions about the world around the dungeon or dungeons. You deal with issues of theology and economics and culture outside of "Can I get healed in town at the church?" and "Can I sell a karkadannan horn in town?" and "Does everyone walk around without pants in your tribe Bjorn?"
When these questions happen, choose the most shallow answer. Those answers should be yes, yes and it's worth $x, and yes. And done.
I'm perfectly serious here.
For a game to stay light and B&P, you can't let it get weighed down with the ballast of inter-church politics, problems of wages vs. coin availability, or cultural nuances between barbarian tribes. The more detail you put on there that isn't a simple answer that lets you get back to the reaving and the slaying and hearing the lamentations of their door hinges as you kick the doors down, the more your campaign gets away from that. This is why my campaign basically has one megadungeon and one town. Oh, sure, there are off-screen places and other dungeons the players can go to. But I channel everything into a small set of adventuring locales to keep the campaign from growing - and inevitably gaining weight and importance that pulls us too far towards Tolkein on the Munchkin<->Tolkein Continuum.
Keep it light. Better "Church of the Good God" and "Evil things live in the dungeon and you can kill them" than complex religious politics and legal issues of subterranean murder.
Confront Expansion Head On
Remember the quandary we had over PCs spreading anti-NPC rumors in town to take the fight from the dungeon to the town? And the solution?
Basically, the players got an idea. I realized said idea would expand the game and undermine a basic simplification of the game. I explained why to the players, and they said, right, let's not do that thing. That's it.
This worked because we pretty simply and directly dealt with the possible expansion of the game from "beer and pretzels dungeon bashing with a safe base" into "dungeon bashing with social conflict on the surface and no safe areas." The former is what we want to play, the latter isn't.
Keep it Fun
Whatever it takes, keep doubling down on the stuff people enjoy in the game. If you introduce an element and it doesn't make the game more fun, take it out or tone it down. Whatever got the campaign started and rolling, keep doing that. Don't let even logical extensions of those elements creep in if they ultimately detract from the issue at hand. Remember the problem of Sequelitis - they're so busy dealing with the fallout of the previous one they forgot what was fun about it (See, all sequels to Pirates of the Carribean). Don't do that. Just repeat the stuff you enjoy and ditch the stuff you don't even if it logically flows from there.
Do this even if you have retcon the things that turn out to be bigger, or more of a drag, than you expected.
Like Minded People
This only works if you have enough like-minded people to sustain the game. And if you're willing to put that vision of the game first. We have that - a core of people who want this kind of B&P gaming. We let people know when they start thinking a little more expansively that this is not the game for that. Generally, that works, because we all know what we're signing up for.
And that's pretty much how you do it. Hold the line, ditch the expansions that take you out of the light and fast elements, and all stay on the same page about what fits this game. 65 sessions and counting. Some players joined, some left, some drifted off, and still others poke at me occasional about coming to try out the game. And it's still as beer and pretzels as it was before, because we don't let it become anything more than we want from it. And if it gets stale, we take a break and come right back to it.