I suppose there is no reason I should have been surprised. Say you knit most of an item, the kind of item that generally comes in pairs, and then put it aside for a year without noting the specific needles used, trusting to your knowledge of what seemed at the time the only possible correct needle size for the job. Say you then picked it up a year later, finished it off with different sized needles (by design) for the last part. It looked great. You washed it and it looked even better, and it seemed a good size too. So you embark on the second of the pair, reasonably sure of the needle size. Partway through you think it is coming out a bit... looser? Could your tension have changed? Do you need different needles after all? But you knit on through this minor doubt.
So then, do you have any right to be surprised when the second half of the pair, after its bath, is obviously - HUGELY - bigger than the first.
I'm still holding a little hope that it will shrink as it dries. I may even have blasted it with the hairdryer on hot just to help it along.
Also it's possible that the second one will be a better fit for the recipient, so at least I can be pretty sure I will be able to replicate it when I knit it again. Sigh.