I have never heard this one. If I had, I'd likely think that her butt was large in relation to her waist. When I look at many ants, their abdomens --"butts"-- are noticeably larger than the thin segment that connects the abdomen to the thorax. If I were talking about some poor girl's skinny behind, I'd probably say "she has a butt like a board" if her butt seemed flat to me. I wouldn't be likely to say this, since my own butt is rather unimpressive. Otherwise, I'd say "she has a small butt".
If you are interested I made the same discussion on the German forum. As they also do not have such an expression (and the original text is the german one) I believe it is an author's "discovery"... I will traduce it literally.