I’ve kind of wondered, since I’m always getting called one — and while not really thinking hard about it, I’m aware of the homologies in male and female genitalia and mentally pictured something discrete and reasonably placed. I guess I must have low self-esteem or something, because science fiction authors imagine something grand and huge and dangerous, instead.
John Scalzi has a sketch of the Kaiju Mangina, and it is awesome and terrifying and silly. And then Howard Tayler drew the Brobot reaction, and it is perfect.
By the way, the drawing is by KB Spangler.