Well first of all, her legs aren't really much bigger than yours. They do, however, have the illusion of being a lot bigger because she is leaner, shaved, and tanned. That aside, there are numerous contributing factors that determine one's strength.
As Mudge said, inherent fiber type plays a role. You must also consider the neural component, which encompasses both intramuscular and intermuscular coordination; the former includes things like improved rate coding, greater fiber activation, improvement in sarcoplasmic reticulum function, and others, while the latter is a fancy way of saying very efficient form and recruitment patterns throughout the body. There are also improvements in energy system efficiency: the storage and usage of appropriate substrate. There are also biomechanical factors that play quite a substantial role. Not to mention, in almost all of the lower body lifts, there are many more important muscles than just the quads.
You must also consider how long you've seen her in action. Do you know for a fact that she isn't just in a phase of her training where she isn't dealing with heavy weights? Maybe you only see her on days when she's modulating the intensity?
Food for thought. Cheers.
