In the 19th Century, many wonderful, exciting things happened. Robert Fulton built a steam-powered ship, and revolutionized sea travel. The railroad engine was perfected, and transcontinental travel became a reality. Internal combustion engines came on the scene, which led to huge advances in travel (and warfare, unfortunately) in the following century.
And, the bicycle appeared on the scene.
The first bicycles were odd affairs, with wooden wagon-type wheels. The frames were wooden, as well, and the front wheel was the drive wheel, with pedals mounted to the front hub.
This eventually morphed into the classic "Penny Farthing", or high-wheeler. Again, the front wheel was direct drive, but the frames were steel and the front wheels grew in size. The taller the front wheel, the faster the bike could theoretically go. A larger wheel will roll further with each revolution than a smaller wheel, obviously. So, at the same cadence, the bigger wheel of two will go faster than the smaller wheel.
At the time that the Penny Farthings were the norm, racers rode the big wheels, up to 60 inches in diameter. I can only ride a 48 inch wheel, myself.
As the bicycle evolved, chain drive came into existence. At this point, a bicycle with two equal-sized wheels of, say, 28 inches could be geared to recreate the rolling diameter of the well-known Penny-Farthing wheels. For instance, a chainring of 50 teeth, driving a cog of 25 teeth gives a 2:1 drive ratio. Multiply that by the 28 inch diameter of the safety bicycle wheel, and you get a 56 inch gear.
In other words, the 28 inch wheeled safety bike, with a 2:1 drive ratio, would move forward the same distance as a 56 inch Penny-Farthing with one rotation of the crank on each bike.
Now, over 100 years later, we still refer to bicycle gearing in terms of "gear inches", the geared equivalent of a certain-sized wheel, even though the vast majority of bike riders have never been on a Penny-Farthing. But, since it is the standard, people just accept that that a "100-inch" gear is a tall gear, even if they don't know why.
Bicycle gearing...Yesterday's technology: Today!