If referring to size:
* Shrunken
* Deflated
* Contracted
* Reduced
* Thin
* Small
If referring to inflammation:
* Healed
* Normal
* Uninflamed
* Subsided
* Soothed
If referring to a feeling of emotional swelling:
* Calm
* Peaceful
* Subdued
* Composed
* Relaxed
The best opposite will depend on the specific situation and what you're trying to convey.