They are usually slightly larger than the cornerback to assist the linebackers in stopping the run.
They can also be seen backing off the line early, to counter the forward motion of a slotback, which is allowed before the snap in Canadian football.
Furthermore, the abolition of the one-platoon system in the 1940s led to a tendency for position names being used on only one side of the ball.
In Canadian football, however, passing was (and still is) a greater portion of the game (due in part to the larger field and one less down in that game) and only one running back was regularly used, leading to the offensive distinction between halfback and fullback eventually becoming obsolete.
The rough equivalents of the halfback position in American football are the strong safety and nickelback.