From a visual point of view, a design doesn't have to be symmetrical to be balanced. Balance has to do with the weight of elements on both sides of an axis (or multiple quadrants). Check this other question for example. Elements with different sizes can still be balanced:
You can also have an even number of elements that have the same size, but use other techniques to emphasize the call to action (color, icon, text). As far as I'm concerned, it's 3rd party apps on the iPhone that use CTA with different sizes. iOS uses the same size for all buttons, right?
From an usability point of view, I'd say it's probably more difficult to tap something that is either surrounded by elements and without enough space between them, or in the end closest to the finger.