Why are bicycle wheels the same size?

[siddharth]

Why are Bicycle wheels the same size?
Is it because it is easier to manufacture and maintain or does having same wheel sizes in the front and back offer added stability when compared to having a bigger wheel at the back (like the early models)?

 

[Danger]

There is no reason other than perhaps the lack of having to keep more than one spare tube around. Lots of bikes have a smaller front wheel than rear. At least, they did when I was a kid.
For an extreme example of the opposite situation, look up penny/farthing bicycles.