very true of what said above, but starting to see 23in+ wheels.... they're HERE, 30+ in wheels are here! Pointless if you ask me. Just a fad like the "donk" cars and "cen-cal" lifted trucks that run big wheels and rubber band tires, hopefully it'll die out soon. But its hard to stop a fad that says "look at me, I spent THOUSANDS PER wheel!"It has to do with a lot of things.
The motorcycles particular intended purpose, frame geometry, front end rake/trail, all play a part. Add to that gyroscopic stability, weight, and greater/lesser rolling resistance....and appearance/style comes in a distant last.
Machines that travel off road will usually wear something close to 21" up front. This lets the tire roll over bumps and ruts more easily. It also allows the front to "lead" the rear better (so to speak) Smaller front wheels have a tendency to push (understeer) in low traction turns. If you want a bike to handle well when power sliding, you'll put a larger diameter wheel up front. (the current drift fad, not withstanding. lol)
Sport bikes often have the same size rims, front/rear. Again, has to do with stability in the intended environment. Cruisers are low performance machines, so style and appearance will usually overcome good handling manners. This is why we're starting to see these ridiculous big (23" and larger) rims up front.
There is more to it than the above, but this gives a basic description without writing a book...
I personally LOVE the 17/19 set up on my bike. front isnt too big looking and it still handles great.