What is the antonym of "outgrow"?

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.