Not all words have antonyms that have a one-to-one correlation.
Basically, you have to say it in a phrase. You can say you've dropped down into your older clothes. Or have lost a size and are back into your older clothes. But I don't think there is another word for "undergrow"
I'm looking at Pint Sized Terror's list of words and so many of them have a negative feel to them in the english language. Maybe part of the retraining of our brains is the acceptance of less being an ok thing.
Eliana ~ go for it! It's one of my favorite I Can Haz Cheeseburger titles, one I relate to a lot lately.