You could very well be correct on everything you said.
However, keep in mind, I've seen other people reference different pics--from those exact same tournaments--they came from, to illustrate how skinny and "fit" Petra was. So once again, it just shows you how different outfits, their fit, colors, and picture angles, can affect someones appearance.
And of course, none of this let's us know how well prepared, trained Petra's body and tennis is, along with her head, confidence and mental frame work.
They're all factors. Cause Petra, is not one of those top players (not at the moment, until proven otherwise), who can/will give you expected performances or effort in certain/set areas.