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!
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There is no single word antonym in that situation (at least, none I am aware of).
The idiomatic phrase to use would be:
I lost so much weight that I had to tighten my belt!
or for another article of clothing, perhaps:
I lost so much weight that I had to take in my pants.
I don't think outgrew can be used with belt, i.e. I outgrew my belt.
I think what you are looking for is:
I lost so much weight that I had to tighten my belt.
and to explain the original situation:
I added so much weight that I had to loosen my belt.