Marshmallow in the vacuum chamber just after pumping the air out.
It grows due to lowering the pressure in the chamber. Air in the marshmallow is pushing outward expanding it.

Marshmallow after being in a vacuum for about 15 seconds, air is being pumped out of it, and it begins to shrink.

It continues to shrink after about 45-60 seconds of pumping.

Vent or let air into the chamber which applies pressure once again on the marshmallow and it shrivels and shrinks.