Marshmallows are made of sugar foam with lots of tiny air pockets filled with air. When you put the marshmallows in a jar and remove the air inside the jar (creating a vacuum) the pressure inside the jar decreases and therefore the air molecules inside the marshmallows want to expand. You will see this because the air tries to get out of the air pockets and causes the marshmallow to get bigger. The marshmallows will shrink again when the vacuum seal is broken and air rushes back into the container. I assume that the marshmallow will not go back to its originally size because many of the air pockets will be broken and the air molecules will not be able to get back into the marshmallow. This should actually result in a smaller marshmallow than the one you started with since the full atmospheric pressure is now pushing on the marshmallow.

Take the air out of a marshmallow -- and you get a tiny bit of tough plastic material. (I have done it in a vacuum chamber). Unfortunately, I do not know how to put it back in.