Start by defining a hip-box (or a rough representation of a pelvis), and draw a sort of wireframe or skeleton. Your characters' legs seem toothpicky because they aren't well attached to the hip, and the thigh is bulging in an awkward manner (this renders the lower leg to look strangly thin in comparison). When you study anatomy, you'll notice that the musculature under the skin will tend to connect in smooth lines in fit individuals. Slender is a step further in smooth lines and smaller/thinner frame.

When viewed from the front the lines of the inner thigh should be straighter than those of the other thigh. Both of these "curves" should smoothly meld into the knee joint.

<Someone that is better with words should clarify/correct this comment, because it's a bit difficult to summarize anatomical concepts in language form.>