I asked my brother this question and got his answer this morning. It is just too good not to share.
"I’ve conducted extensive research into this very question and have discovered the answer. The size of a man’s “empty” box is in direct proportion to the size of a woman’s “memory” network. The longer a woman’s memory’s, as in “do you remember what you said to me on our 7th date, 21 years ago…” and her expectation on a man’s memory of such events, the larger a man’s “empty” box!"
I just can't stop laughing on your brother's answer, it's just so hilarious