Well, it appears I did rescue the wrong jumper, but on the other hand the one I rescued now seems to be wearable. Perhaps it was only temporarily shrunk, and expanded again on contact with water.

Actually I'm quite pleased about that, because the jumper I'm now wearing doesn't have a hole in it whereas the other one did; so if I had to choose one of them to be wearable, it'd be this one. But I'm confused and puzzled as well; I've never heard of clothes un-shrinking in the wash before…

So in fact I still can't work out whether I'm a complete spongehead or not, given that it all seems to have worked out to my advantage. Perhaps I'm an incomplete spongehead. I wonder if that's worse.