I was inspired to look up balance on bicycles after we bought into the "balance bike" trend, and it actually worked! Here's a video (ignore my excited yelling at the end) of our two year old balancing on her teeny-tiny bicycle. I never expected that she would pick it up so quickly.
I guess, when I think about it, there are two main aspects to learning how to ride a bicycle:
I learned by pedaling first, then, when I was older, learning how to balance (and falling quite a bit). Now that I've seen it in reverse, I'm amazed. Of course, balancing is probably easier to learn when your center of gravity is closer to the ground. It also may be easier to learn to balance on a bicycle when you are still learning about balance when walking.
Here's a summary of some research I found from UC Davis, studying balance on bicycles: