A "stealth bust" or "undercover bust". This describes the delightful surprise you get when a woman's breasts appear to be average or small when she's normally dressed, but then actually turn out to be unexpectedly large when she's naked. Also known as an "FBI visit", "CIA doorknock", or "Mossad strike" (or indeed any organization known to be stealthy... so not the NYPD, presumably).
AC (who is female): So, Bing, how did your date with that hot Indonesian chick go?
HMB (who is male): I'm still dazed.
AC: What happened?
HMB: Well, we caught a movie and then had dinner. After a glass of wine she asked to see my apartment, so I took her back. And you'd never believe it, but when I got her bra off, they almost poked my eyes out!
AC: Dear me. A police sting?
AC: Get out! She looks so petite and slender!
HMB: You've got that right. She's the last person I'd have suspected of smuggling grapefruits. But hot damn! It was like dead heat in a zeppelin race in there.
AC: How big are they?
HMB: I'm guessing about 1.7 to 2.1 British Standard Handfuls. Not sure though. I might have to go back for more testing.
AC: Yes. Do that. Now.