if the surface area of alveoli is 75m square then why dont we swell this much when we breathe?

You are not taking it in the right way.

Its absolutely correct that surface area of alveoli is 75 m square but their stucture is such that it increases the surface are but not the volume. They are numerous spherical structures (refer to the diagram) which increase the surface area for exchange of gases but the volume remains same.