I confess this is less a question people have been asking and more just confusion that just keeps happening.
Of junk storage
In my checking to make certain this "trunk" expression meant what I thought it did, and not something more that I naively did not know about, I was informed that it is primarily an expression in the United States.
Therefore, I wish to assure everyone unfamiliar with it in other countries that it has absolutely nothing to do with the size of one's rear end, and that we Americans are quite cultured and serious and above saying such things in such ways.
Besides, what difference would junk make to the size of a trunk? Vehicle trunks do not expand as a result of being full. That's simply not how they work, so how could that possibly be what the expression refers to? It cannot, obviously.
What it means if not that is obviously not important.