Yeah, that's how I heard of it, large breasted women. I still don't get the analogy, however.Its the first time that I've heard that one, although after a quick google search it appears to be associated will a large breasted woman.
May be it is something along the lines of: "You don't look at the mantelpiece when you are stoking the fire." Except its with regard to her personality rather than her physical attractiveness.
Was it use like this?
Guy1 - I really like this girl's personality, but she has small boobs.
Guy2 - You can't motorboat a personality.
And in this case "motorboat" means what?