The formula you cite, battery capacity x "C" rating, tells you only that the battery is capable of providing up to 66A of current. That doesn't mean the power system will USE 66A. It only means that according to the battery manufacturer, the battery can provide that much.

You did not say whether the battery used when the ESC was damaged is a 2-cell or a 3-cell lipo. If you look at the specifications for the motor, they say the recommended prop on 3 cells is 8 x 4. Were you using a 3-cell lipo with the 9 x 4 prop?

For a given motor, a large prop will draw more current than a smaller one. If you used a larger-than-recommended prop, you may have caused the motor to draw excessive current, which in turn was too much current for the ESC. Whenever you depart from the manufacturer's recommendations, you are into the realm of experimentation. A wattmeter is a mandatory piece of equipment when you are experimenting with electric power setups. Without a wattmeter, you are merely guessing.

- Jeff