Is it really just as simple as the number of receptors? Seems to me that not all scent receptors are created equal. I would imagine all critters have higher sensitivity to some smells vs. other critters.
For instance our nose is really sensitive to rotting smells (sulfur and amine compounds). Ethanethiol has a strongly disagreeable odor that humans can detect in minute concentrations. The threshold for human detection is as low as one part in 2.8 billion parts of air. They put this chemical (ethyl mercaptan) in with natural gas pipelines so we can detect leaks (that sewer smell).
Cadaverine, skatol, and putrescine are also detected at minute concentrations. Great names or what?