I have heard the opposite. chances of passing from the big cities is better than in small cities. you can yourself compare the result of any small city and big cities.
I myself appeared 3 times from a small city and failed while the 4th time when i appeared from a metro i passed and got the marks that were really beyond my expectation. this may be because scaling is not done in the result of big cities where there are better job opportunities for CA's than in small cities. ( This is just my general obsrvation)