when a nurse gives a shot in the muscle, one consideration is amount of medication. If it is a small dose, you can put it in the arm (deltoid muscle)
If larger or you need to do z-track, it needs to go into larger muscle. Gluteus medius (buttock) vastis lateralis (hip) or quad.
You can cause damage to the muscle if large volume in small muscle.
Sometimes it is safer going in the hip if someone is combative or fighting. There are various reasons why someone would get an injection in one site verses another