Not impossible! Famous story ive posted here before, one of the gang of hounds we had when i was younger, and very young at this point, was Olliver a rather ancient white pekingese..who when being walked by my sister and friend, was greeted by an in-season golden retreiver bitch, she lay down, he hopped on and before it was noticed *dogs were ahead* the deed was done, and yes she had pups! it's so many years back now I can't remember what happened too them, I'm sure a neighbour ended up with one but i remember being told they were quite odd! Pomeranians x Labs get advertised a LOT
Seeing how our rottie x GSD was in season, she would sit/lie down for a dog. A chihuahua could easily have mated her. If the bitch is the bigger, then anything is possible. A bitch in season knows no bounds to tartiness (IME anyway ).