What makes marshmallows so fluffy is that they are mostly air. There are thousands of tiny holes in marshmallows that are filled with air. Materials like this are known as "porous" because the holes are referred to as "pores". When you squeeze a marshmallow with your fingers, it becomes smaller because you are pushing the air in those pores out and making them smaller. Then as you let go, air goes back into the pores and the marshmallow will expand.

So, if you were to put a marshmallow in a vacuum environment, all the air would come out of the pores and it would shrink.

I hope this answers your question!