why did the first bicycle have a huge wheel?

- Maybe it will be a very old bicycle...

That type of bicycle was called a penny farthing and the big wheel was to make it easier to pedal. The modern gears on a road or mountain bike do the same thing, just in a much smaller package.

It operates on the same direct-drive principle as a toy tricycle, only with two wheels. It's simple and was part of the evolution of the bicycle.