What was unusual about the penny farthing bicycle?

the front wheel was bigger than the back wheel
The most unusual thing about the Penny-farthing bicycle is that it was a high wheeler. The front wheel of the bike was very high while the back wheel was very small.
The penny farthing ( also known as the high wheel, high wheeler and ordinary) has a large front wheel and a small rear wheel. The front wheel is directly driven by the pedals. It took a lot of skill to ride the ordinary without coming to grief.
There was a very large front wheel and a very small rear wheel. There was a direct drive from the pedals to the front wheel. The rider was seated high and had to learn the technique of starting and stopping the penny farthing - which was also known as the high wheel, high wheeler or ordinary.

It was called the Penny-Farthing. Those were two different sized coins. The front wheel was large, the rear wheel very small.