The resistance provided by the water makes swimming an ideal environment for toning your muscles, especially your back muscles. In addition, swimming tones your core muscles -- the front and back of your torso, bottom and tops of your thighs. This makes your waist appear thinner and your back wider. Some strokes focus more on the use of the latissimus dorsi, the major back muscle group that flares out from your waist.
When you swim, you engage most of the major muscle groups in your body. The legs power your kick, using the quadriceps at the front of the thigh, hamstring at the back of the thigh and both soleus and gastrocnemius muscles in the calf. Your upper body is heavily engaged, using the shoulder, chest and back muscles, particularly the latissimus dorsi, deltoids and rhomboids as well as the trapezius and teres minor and major in your back. The arms are engaged as well, both the triceps and biceps power your arms movement through the water.
The backstroke engages your back muscles because you need to fully extend your arms and rotate your torso to power your movement through the water. The butterfly requires pulling your arms through the water under your body, engaging the latissimus dorsi and other back muscles significantly during the stroke. During the butterfly kick, your core powers your body roll, enabling forward movement as well as the upward movement to breathe. Freestyle also requires powerful back and arms muscles, as well as a strong core to maintain body position and roll through the stroke. Breast stroke does engage the back. The start of the stroke emulates a reverse fly against the resistance of the water.
To further enhance the development of your back muscles, use a pull buoy and pull paddles. The pull buoy removes the legs from the stroke as you hold it between your knees or thighs to keep your legs afloat, using only your arms to move yourself down the lane. To add a cardiovascular component to a pull drill, change your breathing pattern. Breathe every three strokes for one lap, then every five strokes for one lap, then every seven strokes for one lap.
If you don't want to increase the size of your back muscles, avoid pull drills and focus work on kicking drills with a flutter board instead. Aqua running engages your lower body and can be practiced with a flotation device to minimize the use of your back muscles.