MacBook – Using 85W MagSafe inplace of 60W MagSafe 2 for MBP retina 13″

Most switching power supplies are most efficient when operating at 50% capacity. And they're definitely pretty inefficient at less than 20% capacity.

Since the MacBook draws less power than a MacBook Pro, I would suspect that an 85w power supply would work just fine, but the larger power supply for the MBP probably would draw more power from the wall socket than is necessary when connected to a MB because it is operating in a suboptimal capacity range.

Here's an example: Take for example the 45w power supply for the MacBook Air. Sure you could hook up the 85w power supply, but since the MBA might only draw 20 watts, the power supply is operating at less than 25% capacity. At this capacity, the power supply may only be 60% efficient (an educated guess), that means the power supply has to draw 33 watts of AC power just to supply the laptop with 20 watts of DC power.

Long story, short: It's a power conversion efficiency thing. I'd say buy the one that is designed for your laptop so that you save energy.

Apple's official word on the matter is:

Make sure the proper wattage adapter for your portable computer is used. Select the appropriate power adapter for your Apple portable computer. You can use a higher wattage power adapter, but you cannot use one with less wattage without potential operating issues. (here + discussion here).

So your 13" can use your 15" charger, but not vice versa.

I've never heard of it voiding a warranty (nor experienced it when we've used the wrong charger), but it's better to be safe than sorry.