With a diet balance is the key meaning that you need to coordinate your exercise program with a good diet that will help you become lean a lot more quickly but will also give you the energy and nutrients to help you rapidly recover from your workouts.
Without the proper diet, actually, a proper eating SYSTEM it is SO easy to get fatter when doing workout regiments that are more intense. More stress, even if brought about by exercise usually ends up creating more fat and general non muscle weight gain.
A good diet helps you rapidly recuperate from your exercise workout without having you gain excess body fat and without making you feel like you're lacking. A bad diet or no diet will likely slow down your progress in exercise or completely reverse your progress by overcompensating for the "strange discomfort" that comes with an increased metabolism.
But you need that diet, that eating system that will allow you to intake larger amounts of nutrients, efficient nutrients, that will satisfy the physical body, that will make you feel good, that will provide you the fuel, the energy for you to exercise more and have lots of extra energy to get work done WITHOUT making you fatter or 'more bulky'.