I assume that you mean without any more effort than normal. Breathing is a muscle contraction of the diaphragm, so it does require some effort. I don't know the specifics of who you are, so I can't say with assurance why you are breathing deeper without any extra effort.
One possible reason is simple aging. As you grow and mature, your chest cavity enlarges as well as your lungs. Your breaths are deeper because you have more lung capacity. You don't notice any extra exertion, because the diaphragm muscles have also gotten larger with aging.
Another possibility is that you have become more efficient at moving the air, so you are moving more air with the same amount of effort. It sounds the same as the above example with getting older, except this time the cause is different. If you have been doing a lot of cardiovascular training (running, swimming, biking, etc.), you are training your body to do more with each breath. That means more air per breath without feeling like it is extra work.