Here muscles are nowhere near the size of Williams. Her muscles look like a womans, when that woman is in shape. She may also be on something, but it's not as obvious as Williams.
You can't build muscles the size of Williams without taking steroids as a female. It's biology. That simple. Her biceps are larger than guys who actively try and build muscle while taking tons of supplements. That is not natural for a woman.
If you work out, and are involved with any type of serious mass gaining. You understand what she has is not possible without the help of PEDS for a woman. Tennis also has a pretty much non-existant, horrible testing program. It's so obvious she is on something, that it blows me away people can't see it.
You can also see how much larger her muscles have become over the years.
You can also see, at one point, she was on HGH. Maybe just to rehab injuries. Man or woman, when you take HGH for an extended period of time, your stomach looks like this.
This girl here:
Worked out just as hard as Williams, but was caught on steroids bringing some of her hormone levels higher than a mans. Her arms are significantly smaller than the "natural" williams.