I have saw the answer from Apple Store before, hope it can help you.
The power (Wattage) requirement of the Macbook Pro is higher (the adapter is rated 85W, while the one for the 13"is rated 60W), so you must use the 85W adapter. These regulated power supplies are able to maintain the voltage (V) constant, while the current (I) is adjusted according to the needs of the equipment being powered. Since P=V x I, if V is kept regulated at 10 Volts, the maximum current supported by the 60W adaptor will be 6 Amps, while the 85W adapter will supply up to 8.5 Amps. So, using the 85W with the 13" does not constitute a problem (on the contrary, the power adapter will run cooler), but if you use the 60W with the Macbook Pro, the current drained from the adaptor will exceed its specs, the adapter will overheat and its life spam will be shortened considerably.