Physical and Mechanical leavening
By physical leavening we talk about the availability of liquids. What happens is upon heating of bakery products during proofing or baking water and other bound or unbound moisture will expand: the density is changing. When the dough becomes warm enough, water will start to escape from starch and protein cells inside the dough and turns into water vapour or steam. Upon continuous heating this steam will expand, until it escapes the dough matrix or is assimilated by one of the starch or protein cell upon gelatinising or denaturing. During this stage of expansion and fixation of the structure (gelatinising and denaturing) the density continuous to change, gas expands, cells expand and volume increases. In certain cases it can even lead to an increase of close to 15% volume increase, this however only happens in product stabilisation phase of baking: so a certain amount of that volume you will loose upon product cooling.