if you were any closer, it would bite you

If you were any closer, it would bite you is an exaggerative and sarcastic way to say that you are much to close to something or someone. This phrase is most commonly used for something that has the potential of being harmful or dangerous if yu get too close such as a fire, stove burner or even a wild animal. In the case of the animal, this person is speaking literal in that you are too close to it and risk being bitten.