When a marshmallow is microwaved, the microwaves heat up the air inside the marshmallow. When the air inside gets hot, it gets bigger and pushes out the sides of the marshmallow making the marshmallow puff up and get bigger. If you keep microwaving long enough, the outsides start to melt and the hot air inside escapes and the whole thing turns into a melted pile of goo.

Marshmallows have a lot of air in them. When that air heats up in the microwave, it takes up more space, pushing out on the marshmallow and making the marshmallow grow bigger.

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Marshmallows are a foam of sugar and are full of air. When put in the microwave a small amount of water trapped inside the air pockets heats up and expands which pushes on the sugar making the marshmallow get bigger. When the microwave is stopped the air cools off so the marshmallow shrinks back down. If you microwave the marshmallow for a longer time the air will eventually escape and the marshmallow will collapse and start to melt.