I agree with you about Serena. She is just a massive woman. I'd be surprised if she was using.
Stosur though - I've had a question mark about her ever since she was playing with Völkl, which is quite a while. She's gone through her 'dental braces' phase which a lot of GH users seem to go through, when I saw her up close I was surprised by how thick her skin appeared (granted - she spend a lot of time in the sun) - and how large her hands are. Tennis today - especially women's tennis, is all about the grind. You have to stay out there and hit and run and hit and run some more.
The first thought I had when I stood next to her was "How can she maintain this much muscle if she's doing all that running every day?". Maybe I'm wrong, but I've been around a few women who use to get ready for competition and she looked exactly the same to me.