[RPG] How to tailor an encounter so that a low level, mostly mundane party can defeat a wasp swarm

I proffer this answer only because @Zach doesn't like the answer provided above which, from a system point of view at least, is more consistent and easier to implement for the GM. However, here are some other options to work around the inherent imbalance of monster races without @Gates answer:

First and foremost - try to consider balance to be more than the sum total of what the system has to offer (and especially the combat system). In the example of the ogre you could:

1. Make it apparent that life is difficult for creatures that big when pretty much everything is made for smaller folks: "Er... yeah - I repair weapons but something that big? I don't even know where to start. Leave it with me for a couple of weeks and I'll give it a go but don't expect anything quick or cheap"
2. Play up reactions to the racial stereotype. There's likely to be settlements that just aren't comfortable letting an ogre wander around them.
3. Magical items - in a campaign where they're relatively common how many are they going to find that actually fit an ogre?
4. Transport: horses, coaches, boats etc are all likely to be difficult propositions.
5. This one takes a little more work - but you can provide a slightly different perspective on the world with some effort to re-describe things for a less advanced race who might not understand the technical or social nuances of other species.

All of the above take more effort than just throwing in some checks and balances at character generation but they can add a lot of atmosphere to the game. I would also strongly advise talking with your group about whether any of them would help provide the sort balance that you want - after all there's not point putting balance mechanisms in that either don't work for your group or make the race entirely unplayable for your players.

Polymorph Any Object yourself into an Animated Object, with that object being an appropriately sized light weapon for a small creature. Light weapons are objects two size categories lower than the appropriately sized user, so you are now Fine -2. With Extra Item Slot and a Belt of the Weasel (not a polymorph effect), you can additionally fit in places designed to accommodate a Fine -5 size creature without squeezing and can slip through spaces a Fine -7 creature could move easily through by squeezing. Assuming the halving per size category rule continues below fine, this means you can squeeze through cracks 47/1000ths of an inch wide and walk easily through spaces 187/1000ths of an inch wide. For reference most credit cards are ~30/1000ths of an inch.

Alternatively, Polymorph any Object into whatever you dang well feel like; there is no limit to how small you can become with that spell, just as there is no limit to what kind of creature you may turn into.