Poori is an Indian bread that puffs when it is hot. When measuring the kezayit olive size of bread that is the minimum to require Birkat Hamazon, does one include the volume of air in the food, or does one have to subtract this from the measurement?

(I'm using bread as an example. But my question would similarly apply to other "airy" foods like borekas, croissants, puffed wheat, etc.)