that girl is absolutely right, bad posture is a self defence mechanism; and it is a biological one, innate in all of us. It is based less on personal reasoning, trying to make oneself invisible, than it is on our nature. To make oneself smaller than another person is a gesture of submission to that person, and this is seen in all people around the world, and in animals too. By the same token, to be larger than another person is to be in a dominant position.
"... in submitting to another one makes oneself smaller. In detail this can happen by means of prostrating oneself, kneeling or bowing."
Bad posture makes a person seem weak or unassuming, "... one makes oneself small, and this is the precise opposite of threatening behaviour." As well as going unnoticed, they want to avoid confrontation, and are willing to surrender before one even begins. They are sure enough of their own inferiority that they think it better just to lose than to be hurt in conflict that they will surely lose regardless. A person who stands straight and tall is showing their readiness to fight, to prove their worth, and to assert their power over others. They are confident of their strength and prowess, are broadcast this to everyone around them.
Showing that one is submissive triggers emotions and behaviour patterns in others; sympathy, soothed aggression, and nurturing behaviours. These are just reactions to stimuli, no different from a mother's anxiety from hearing a baby's cry. So animals, humans included, take full advantage of these behaviours by acting in ways that will trigger them. The sight of a child inhibits aggression just by its presence, and many cultures take advantage of this: "... if the Australian aborigines wanted to make contact with the white men, two high-ranking men would push a small child in from of them... they relied upon the fact that no-one would do anything to harm a small child." Equally we all trigger behaviours in other people with our actions; we smile to put friends at ease, we shout when we are angry, we put an arm around someone's shoulder to make them feel safe. We stand tall and slouch low to tell other people where we stand in the social hierarchy of power.
It seems very intuitive, because it is just how our brains are wired. It is apparent in our language: to "act big", or to belittle a person, are both related to power, domination and submission. The "big city" is scary and unnerving, it stands immense and immovable as we cower beneath it, it dominates every part of our being. The way we nod when listening to someone speak, we are momentarily making ourselves smaller, submitting ourselves to their ideas.
For a person standing upright, folded arms serve to make the chest seem larger and more powerful, the elbows are pointed forwards to make the most of this. For a person hunched over, the lengths of the arms are held tightly to the chest, so as to make the front flat, and to hide the hands. The hands are a seat of power; the fist raised in the air, the spear or the knife clenched with white knuckles, the outstretched hand to show that it is empty and thus inviting. To hide the hands away is to remove one more means to attack, and so to make oneself that little bit more vulnerable. "Helplessness, weakness and child-like behaviour equally arouse pity."
Iguanas are an animal far removed from humans, but even they use size as an indicator of submission. When they fight, they will head-butt each other until either one of them has been pushed from a sought-for piece of land, or when one "... realises that he is not a match for his opponent ... and lies flat on his belly in a submissive posture in front of the victor."
I think that this role of body size in displays of submission and domination are linked to the "short man syndrome"; a man who feels constantly weak and dominated will subconsciously strive to overcome this with a dominating, competitive and aggressive personality. For the same reason, tall men are most often calm and kind, "gentle giants", because they have no need to be aggressive towards people who are automatically made less powerful than them. Tall men speak softly and are self-confident, short men speak forcefully and are insecure, all because of the role of size in our perceptions of power.
Quotations are from Love and Hate (originally Liebe und Hass) by Irenäus Eibl-Eibesfeldt, translated from the German by Geoffrey Strachan.