Carabiners are one of the most important pieces of gear when it comes to climbing. They play a large role in the force distribution of the physical system as the climber scales a route. For this reason, carabiners vary in both strength and size depending on the type of climb being done. The most common types of carabiners used in both top rope and lead climbs also vary between the belayer and the climber.
    A few common physical features of carabiners being used today are: auto lock features which ensure that the carabiner will stay in the locked position (used typically when belaying), size variations in accordance with the amount of equipment that is being clipped into the carabiner, and variations in size in accordance with the amount of force the carabiner needs to withstand (the bigger and thicker the carabiner, the stronger it is).

  
the above image was obtained from: http://www.rei.com/learn/expert-advice/carabiners.html

Example Test performed by the UIAA for carabiners with an auto lock feature:

the above image was obtained from the UIAA: http://www.theuiaa.org/safety-standards.html