Pumping the air out of the bottle changes the pressure inside the bottle. A plastic bottle would collapse in on itself but the solid glass bottle is strong enough to keep its shape. Marshmallows are full of tiny air bubbles and when the air is pumped out of the bottle, the bubbles are at a much higher pressure than the surrounding air. The air in the bubbles pushes outwards, the air molecules move further apart, the bubble gets bigger. We see the marshmallow stretching and growing larger.