As succinct as it sounds: what is closer to the camera lens appears larger! The effects of this are extremely important to understand, especially when photographing people.
The selection of the object and the focal length also makes a difference to the extent to which the effect is visible - but basically this effect is always there.
In plain language: I can inadvertently enlarge parts of the body that I might not have wanted to be so large. On the other hand, the photographer can deliberately use this effect, for example, to make legs look longer, emphasize the décolleté or accentuate upper arm muscles.
Let's take a look at the effect in our example photo. The only difference is that our model is standing on the front foot (left photo) and on the back foot in the second photo. This means that in the left photo, everything from the thigh and upper body onwards is closer to the camera - whereas in the second photo, these areas are further away from the camera.
Effect: the thigh is much thicker (white square) than in the right-hand photo. Here we can see the white square of exactly the same size and how the thigh looks around 20 percent smaller (or the other one bigger)!
The head also looks different in size when comparing the photos. Please always remember the principle - what's closer looks bigger - when taking photos!
Depending on the choice of lens, the effect will be stronger or weaker when taking photos. For example, at small focal lengths such as 27 mm, the effect is very quickly and dramatically visible - at focal lengths of 200 mm less so (but also present).
Of course, you can now split hairs and say quite precisely that it is not the focal length, but the distance to the subject. If I want to photograph the same image section with a 200 mm focal length, then I have to move significantly further away from my subject than with 27 mm.
It is important that the photographer internalizes the understanding of this magnification effect and doesn't pore over their photos afterwards. This is why the 50 to 80 millimeter focal lengths are so often recommended for portrait photography. However, you can also take great portraits with any other focal length - you just have to master the tools of your trade.
The following is a targeted use of the "what's closer to the lens looks bigger" effect. The hard-worked muscles are shown here. They look even bigger and more impressive when the upper arm is closer to the camera and a short focal length (27 mm) is used:
To do this, the upper body is tilted towards the camera and the arm is held specifically towards the camera.