Telephoto (zoomed in) lenses tend to compress objects and make them look closer to each other.
Wide angle (zoomed out) lenses tend to stretch objects and make them look further way from each other. Objects very close to the lens can become distorted. Therefore, portraits (head and shoulder) are usually made with a slight telephoto lens. A wide angle lens would distort the nose and make it look larger than normal.
The depth of field (what is in focus close to and far from the camera) is greater with a wide angle lens. Backgrounds are more likely to be blurred with a telephoto lens. This is a second reason telephoto lenses are often used for portraits.
If the same subject were photographed with a telephoto and a wide angle lens and the size the object were kept constant, the amount of background (objects behind the selected subject) would be less with a telephoto. This is a third reason telephoto lenses are often used for portrait photography. Portrait photographers usually prefer to have non-distracting backgrounds.
Wide Angle (Zoomed Out) — Note slight curvature (distortion) of the bench
Telephoto (Zoomed In) — Note less back ground and more blurred background–the wall behind the bench also appears closer
Wide Angle (Zoomed Out)
Telephoto (Zoomed In) — Note how much closer the second cow and red object in the railing appear