It's easy to say that you outgrew your clothes, but what's the word to indicate the opposite?
I lost so much weight that I _ _ _ _ _ my belt!
There is no single word antonym in that situation (at least, none I am aware of).
The idiomatic phrase to use would be:
or for another article of clothing, perhaps:
I don't think outgrew can be used with belt, i.e. I outgrew my belt.
I think what you are looking for is:
and to explain the original situation: