The 6ft 6in 23-year-old came off the bench to head home Jonathan Grounds' 64th-minute corner for a fourth goal in this season's competition, all of which have come against top flight opposition.
Smith has only netted three times in League One this term, but his imposing physicality asked difficult questions that round four victims Liverpool and city rivals Everton struggled to answer.
"We don't really see anyone like that apart from Peter Crouch," said Everton assistant manager Steve Round after the game.
"Whenever the ball's in the air, if a goalie's knocking the ball in the air from 75 or 80 yards away, if you're 6ft 6in then you've got a chance of heading it.
"You've got to compete with that like we did and make sure you pick up the second balls and flicks.
"He scored a good goal from a corner again."