Apply the Scale (CTRL+A in Object Mode) of both the propeller and Empty!
If you have an Object Offset with the Array Modifier and that Object has a (uniform) Scale of 2, then each ‘iteration’ of the array gets twice as big as the one before…Might look a bit unintuitive at first, but that is actually the power of the Object Offset.
Don’t worry, almost everybody struggles with that;)
Feel free to ask more, if you still don’t get it!