Why Giant Wheel have Giant Wheel?

Curiosity which led me today to ask this stupid question, Why the old time bicycle have a bigger wheel than the other, don't worry not because of hipsters back in these old days.

Before dodging into this matter here is a picture of what is used to be called "Penny Farthing"

Yes, Penny Farthing is the name of this giant wheel just imagine a dime rolling after a 50-cent piece and you'll get the idea.

So, why the Giant Wheel have the Giant Wheel in front?
Speed, that's all folks.
and to be in the same height as the horses of course.

The below are the uses of this thing nowadays:

Racing

Good luck and happy farthing.