This would be an awful thing to come home to.
Why it's never a good idea to leave two females together or/and a tiny small dog unsupervised with a large dog.
Your Rottweilers are intact? was the Chi intact too? Were either of the females in heat? or coming into heat?
I have heard of people coming home and finding that one dog (usually smaller) has been killed by a larger dog....but never of one eating it.
Maybe she saw the Chi as prey? The dog would have weighed 5-6 lbs?
In all honesty, I don't think I could live with a dog that killed my other dog and ate it....it would have been at the vets and euthanized.
Gina