You're not going to damage anything by using a higher wattage adater. The devices pull what they need, the power supply doesn't push to the device.

Check out this, it will explain more. http://www.macrumors.com/2012/11/01/ipad...d-with-iphones/

What's odd in your case is that the iPhone draws less power than the iPad will when charging so the iPad charger should have been more than enough for the iPhone.