However, there is so little water and the nature of the dough is such that you don't get the same volume with pie crust as you do with popovers. Air is important for many foods since when the product is heated the gases of air expand to take up greater volume. Beating of a batter or egg whites, creaming of butter and sugar, incorporates air which will help the product rise when the heating begins and the air bubbles expand. The amount of beating has to be sufficient to incorporate enough air yet overbeating will often cause the air to be lost. Air may be the major, if not only, leavening agent in pound cakes and angel food cakes.
Carbon dioxide is an important leavening agent for most baked goods.
Like air, carbon dioxide is a gas that expands when heated. However, carbon dioxide is different than air in that it is generated from within the product rather than being incorporated in by beating. The creation of the gas bubbles itself is enough to provide quite a bit of leavening action. This is apparent with yeast breads, for which it is the yeast that is added to the batter or dough and the carbon dioxide is a waste product of yeast metabolism. As the yeast go to work, the carbon dioxide produced causes the bread to rise.