Models are slim, with practically little body fat to speak of, and have that space between their thighs that gives the impression that they are crossing their legs from the side (they aren't, but it appears that way). The actual reason for this is that it makes them look more slender. As you can see in the picture, she has crossed her left leg over her right, making her look even skinnier. This pose also makes them appear taller and give the illusion of lengthiness. If you look at most catwalks, you will see all kinds of models walking with their legs crossed - especially skinny ones! In conclusion, crossing your legs shows that you are aware of your appearance and want to enhance it by giving the impression of lengthiness or slimness.