Marshmallows are mostly made of air, which is a gas. Gases expand and contract when the pressure changes. Under high pressure, gases can be compressed. Conversely, gases expand under low pressure. An example is helium in a helium tank (you know, the kind you use to blow up balloons. The tank can hold a lot of helium gas because the gas is kept at a high pressure. Once the helium is released into the air it expands and takes up more space than it did inside the tank.

Marshmallows are full of very small air bubbles. (The walls of the air bubbles are made of sugar and gelatin, which is pretty good at holding its shape but is also flexible.)

If you have a good magnifying glass, you can see the bubbles. These air bubbles are filled with air of the same pressure as the air around them when they were made. If a marshmallow is under normal air pressure, the air on the inside and the air on the outside of the bubbles pushes with the same force.

If you lower the pressure around the marshmallow (say by putting it in a vacuum pump chamber), the air outside the bubbles pushes less than the air on the inside. Because the bubble walls are made of flexible gelatin and sugar, the higher pressure on the inside is able to push the walls of the bubbles out -- making bigger bubbles. All the larger bubbles lead to an expended marshmallow.

You can cause a similar effect by putting a marshmallow in a microwave or heating it over a fire. In that case, the heating causes the air pressure in the bubbles to increase and the bubbles expand. Have fun experimenting more with marshmallows and don't forget to eat the results.

Because the air bubble trapped inside the marshmallows expand in response to the lower pressure. This also explains why your ears hurt when you take off in an airplane. The air pressure's lower at altitude, so the air in your middle ear swells like a little balloon, causing pain.

They're full of air (like Styrofoam), and the low pressure means that the difference in pressure between the air inside the marshmallow and the air outside is such that the air inside exerts a greater pressure, puffing the marshmallow out.

Because the bubbles of gas inside the marshmallow try to expand. The higher pressure inside the marshmallow pushes out when the pressure outside is lowered.