Here are some contenders:
By weight:
* Weasels (especially long-tailed weasels) are quite small and can kill rabbits, especially young ones.
* Stoats are slightly larger than weasels but still relatively small and capable of killing rabbits.
* Ferrets are domesticated weasels and can also be quite small.
By size:
* Snakes like garter snakes and rat snakes can swallow rabbits, especially young ones.
* Birds of prey like hawks and owls can hunt and kill rabbits, but some species are smaller than others.
By age:
* Young foxes are smaller than adult foxes and can hunt young rabbits.
It's also important to remember that size doesn't always determine prey. A smaller animal might be able to take down a larger rabbit if it's more agile, faster, or has a better hunting strategy.
Ultimately, the answer to your question depends on how you define "smallest" and which criteria you prioritize.