Shrunken means something has already got smaller, or has been made smaller. It is an old past form of shrink, now used mostly as an adjective in front of a noun: a shrunken jumper, shrunken gums, a shrunken head in an old story. The everyday form after has or have is shrunk — “the jumper has shrunk” — and that is what most people say. Though actually, shrunken still sounds right when you are describing the thing as it is now, especially if the smallness feels a bit sad or strange. A shrunk jumper is possible; a shrunken jumper is the one that looks wrong on the hanger. The word is slightly more bookish than shrunk, but not rare. It always looks back at a change that has already happened; it is not the act of shrinking, and it is not shrivelled (that one is dry and lined, from shrivel).