Right, first we're talking dogs not people so an example using a totally different species is a wee bit pointless, there are plenty of small things that can kill big things, i accept that.
In this case though we're talking a dog and fox, which when it comes to being two different species they are actually in build and 'weaponry' etc are actually fairly evenly matched.
Now the question was not, could a spoon fed, house whippet who is pampered by and old lady, kill a fox?
It was could a whippet kill one?
Now i'd say probably not to the example you use, the same with a molly coddl