What many people do not realize is that every air conditioner also acts as a dehumidifier as well. The moisture that is taken out of the air while an air conditioner is running turns into a liquid form called condensate. Taking as much condensate out of the air is important in terms of the formation of mold and mildew in the room environment.
Now you may ask, what does this have to do with the BTU size of an air conditioning unit? While it is true that a larger BTU size on an air conditioner will cool a room much quicker than a smaller BTU sized unit, you have to consider that the air conditioner while running in a much shorter cycle will not take as much moisture out of the air than if it was running in a normal longer cycle. Hence the possibility of moisture and mildew problems.
Do your homework and get the right BTU sized unit for the area you are cooling, if the unit runs longer it's better for you and the quality of the air you are breathing.