It seems more likely that a free-roaming cat will get rabies than someones 6 lbs lap dog. Especially because cats (at least in my area) don't require licensing or rabies shots like dogs do.
It probably is related to the size of the animals. If there would be more tiger-sized cats and more Chihuahua sized-dogs it would most likely be different.
EDIT: Forgot to say, it don't matter even one bit what size the dog is or how small it is. If they aren't on your side, they're capable and very willing to bark at and attack you.