observation...

What i think happened was the pump started to take put all of the air in the container that held the balloons.  when it comes to the air in the balloons, the pump was trying to take that air but it couldn't escape to the balloons were expanding to take up more space.  when the air was let back into the container, the balloons were able to go back to the normal size.  I think the molecules were expanding and maybe multiplying to fill up the space in the container.