If I have a circuit with 14 guage wire and a 15 amp breaker, can I tee off or extend that circuit using 12 guage wire? It seems that as long as the breaker is sized at 15 amps, the smallest wire size (14) would be adequately protected, and the 12 would be over protected.

Of course that's just me. I have someone telling me that you can never do such a thing. So the question is:

1. is it a code violation?
2. if it is, what is the theory behind it.