As DLane put it, your water usage is dependent on your designated Area size (found in your Advanced Zone Settings) and your sprinkler Precipitation Rates (PR).
For more information, you can view our Support article on How we calculate water usage.
Here's a really handy tip from @Bill.Maupin in another thread on determining your square footage:
We can reset your Water Usage for you, but resetting will not remove any watering done the same day. If you'd like me to go ahead and do that for you, I can do so tomorrow to clear all of your watering so far.