Honestly, consistency is a much prized and proven way to help users navigate an interface. If they see things where and how they expect, they can navigate through easier, find buttons and actions easier, etc.. It's also prized in design and aesthetics. Designers (and even the Android design site) recommend designing to a grid to get a uniform, nice looking appearance. So altering the size of the tiles is considered bad from a UI and appearance perspective.
Funny, because Windows Phone 7 had very static tile sizes (there was one double-width one), wheras 8 allows a large variety of sizes.
They do still fit a consistent grid, though- they're all half or double size another. Android widgets, by comparison, can often be very weird dimensions, and have very different designs that look ugly when combined.
Consistency is not just to avoid confusion and make things easier. Inconsistency is a direct source for irritation and annoyance. Its very easy to see if you give a computer worker a different keyboard setup (that is, other than qwerty or even just a slightly modified version). It drives people nut!
