As anyone who's put marshmallows in the microwave knows, they expand a ton! Sometimes they puff up to literally more than twice their original size (YouTube video for those who haven't seen it).
So, why?
At first I assumed it was because they had a lot of air in them, but that doesn't make sense. There's no way that amount of air can puff up that much from the heat!
What makes marshmallows poof up so much when they are microwaved?
Best Answer
Marshmallows expand so much because the water in them becomes steam, and gas takes up a LOT more volume than liquid. Specifically, 1 mL of water becomes ~1.36 LITERS of vapor, before it gets heated further. That's 1000-fold expansion, before you add additional expansion as the gas is heated.
Marshmallows don't have all that much water content, but when it's trapped in a stretchy gelatin matrix that holds gas readily, it only takes a bit to blow the whole thing up like a balloon.
You are correct that the gas expansion on its own is insufficient; unless I've badly muddled my calculations, gas expansion from 20C to ~150C (caramelization temperature) will increase the gas volume by under 50%.