When the air conditioning is over-sized for a building, you get “short cycling” – the unit turns on and off many times. This causes a lot of wear and tear on the equipment. A properly sized HVAC unit will stay on for a long time, then off for quite a while, etc.
The more energy-efficient HVAC systems have several “stages” – the unit can output different quantities of cool air. The system runs all-out at the hottest part of the day, but when it’s cloudy or not so out outside, a one-output system tends to short cycle. The solution is to have two or more levels of cool-energy output, to match the building requirements.
An analogy is a motor for an elevator. The motor is sized to meet the maximum capacity of the elevator when it’s full of people. You waste a lot of electrical energy with a single-speed motor when there’s only one person being transported. A two-speed motor costs only a little bit more and saves almost half of the operating energy. Likewise with a two-speed condensing unit for a residential HVAC system: pay just a little bit more, but for a more efficient system for the lifetime of the unit.