Short cycling is a common problem that develops in furnaces which are over sized. This happens because the set temperature in the furnace is reached within a very short time and then the furnaces shuts down and it begins operation again after some time. Such a situation continues throughout the day. The result is wear and tear in the heating unit increases which in turn leads to frequent repairs and also the life span of the furnace is reduced.
Inefficiency is another drawback with over sized furnace. When a furnace is over sized it is less efficient as the furnace first fires up it do not reach its peak efficiency until the air that is coming out reaches a steady value. The problem with an over sized furnace is that it spends a lot of time warming up and the result is as it reaches a steady temperature, the thermostat is already satisfied and the furnace shuts off. This causes wastage of energy.
It is highly probable that you end up feeling uncomfortable when you have an over sized furnace installed in your home. This is because with a bigger furnace the home gets too much conditioned air at once and the result is the room might feel too warm and can cause major temperature swings. Moreover, all the rooms in the house are not adequately heated because the thermostat will heat up too quickly and the furnace in turn will shut down before every room in the house is properly heated.