Size does make a difference, but leverage makes a bigger difference. Nurses and other health care workers are trained in proper lifting techniques. They work as a team with other workers so that they don't have to lift dead weight alone. And whenever possible, they use lifting devices to protect themselves.
Lifting is done by using both hands and keeping the patient close to the nurse's own center of gravity. Patients who are too far away from the health care worker are at greater risk for falling. The nurse follows the rhythmic movement of the patient. If the patient starts to fall, the nurse can quickly correct the weight shift and help the patient get back on track.
Sometimes, falls can't be prevented. In that case, workers are trained in ways to break the fall and prevent injury during the fall. Rarely, the nurse may have to let the patient go in order to avoid falling on top of him or her.