A balloon...

The volume of a balloon is determined by 2 things, the mass inside of it and the density of the air outside. If the outside pressure increases as air is put inside at the same rate, the baloon doesn't get any bigger. If the outside pressure decreases with the amount of air inside remaining static it gets bigger, and vice versa.

So your analogy, when you come to understand it, is exactly correct. In the analogy, air pressure outside would be demand.