Firstly, the angle of the shot gives the illusion of more leg room given the focus towards her legs. She is a petite woman and has some room to move back and forth on the seat. She is sitting diagonly from the back right to the front left AND you can see her left ankle sticking out in the walkway - add in the camera angle and you have a spaceous shot.
Secondly, the same women is in the 'exit row seat' image on the same page - you can see her sitting up more although still hunched over a tad so that her head does not go above the top of the headrest. The positions she is sitting in would not be all that comfortable for the average traveller.... the smile is for the camera
I'm sure she wouldn't be smiling if the picture represented a true inflight shot with a kid on one side and an overweight person on the other, taking up all the armrest!