Why is the stop button of a machine larger and more prominent that the start?

A red coloured stop button can be used to either stop the machine normally, or to do an emergency stop in a hurry. It is larger and prominent to make it quicker and easier to smack the button with an open hand. The smaller green coloured start button is either level or recessed, meaning a finger has to be used to start the machine.
The emergency stop button will twist when pushed and stay down, breaking the circuit. It has to be turned to release the button back for normal use and to use the start button again.