This expansion thing is something I've wondered about for years. The phenomenon varies from strain to strain. It can be phenotype, strain, cure, flush, nutes, etc.
I actually think that the smoke doesn't really expand in your lungs. You're just taking in more than your lungs can allow. It's like you don't feel a throat burn, so you don't really think you're taking in too much till- pow, your lungs reject and expel it.
Cannabis is a known expectorant, and some bud has more of that effect than others. When I "cough out" bad it clears out the mucus in my lungs. Anyone have any ideas about this? -granger