A photograph consists largely of curves and straight lines, especially portraits. After all, we see not only the eyes, but also the brain. An important part of the tools our brain uses to capture the meaning of a visual scene is the lines.
Depending on their position and shape, the brain makes an assumption about the three-dimensionality of the figure in the real world.
For example, the classical method - the model rotates the body frame a little to the side and puts a hand on the thigh. The resulting gap between the arm and body emphasizes the bending of the back and makes it look slimmer. If the hand just hangs from the side, then the brain does not see the line of the back, and the whole figure looks wider.
Obviously, the model has not lost weight. But it only takes a second to change the pose and we will begin to perceive her slimmer thanks to the lines.
This is one example of many situations in which changes in the shape and composition of body lines can change the perception of a model. This is not logical, but in this way you make a big difference in how the photos appear and what impression they make. You will develop this over time and add this technique to your tools, learned from experience, if you often take pictures.