What you "see" as the perineum is only about 1/3rd of it, the rest of what becomes perineum during birth is inside the vagina most of the time - that's why perineal massage is done with the thumbs inside the vagina. At the end of pregnancy the whole area "relaxes" so the distance front to back gets much longer. And if, like me, you tore and were not stitched the perineum can look a lot shorter afterwards.

I just measured (hilarious and strangely scientific) and mine (from bottom of vagina to edge of anus) is 1.5cm stood up and 2cm if i lie down. I'm not pregnant though.

Bx