Pullleze do your homework!

I have been around swimmers for the past 30+ years. My female kid was one of them. She had large shoulders and arms from the training. In fact from the back she looked like a guy. So much so that she needed to wear bigger jackets when in training (which was pretty much all year) so she wouldn't tear the seams when she moved.

The training of swimmers causes them to build that kind of muscle especially in the shoulders, arms, and upper back. Most female swimmers wear the tightest suit they can wear to push down their boobs to eliminate drag. But those who aren't well endowed may get that way from being in training prior to puberty and from being lean.

The best swimmers are long, tall, and lean with larger shoulders compared to their hips. This allows for what we call a great streamline in the water which is what you want especially for sprinters. Unlike the sprinters, distance swimmers can be shorter and built like bullets. They are lighter in the water if they weigh less but they often have a different ability than sprinters for endurance. Janet Evans was known to have a metabolic condition that allowed her to swim without building up lactic acid. That allowed her to beat others she competed against even though her strokes weren't the best.

Oh and none of them look all that feminine in goggles and caps.

I could get into the differences in body types for each individual stroke, but ALL of them have muscles in their upper back, arm and shoulders.