It's not hard to make a final-round comeback and win on the PGA Tour. Statistically speaking, it happens with frequency. PGA Tour players are not especially good at holding the 54-hole lead and turning it into a win.
However, making a huge final-round comeback isn't easy. A lot of things have to break right. The leader has to falter, typically, badly. The guy making the comeback has to beat the final-round field average by a substantial amount of shots. Then everyone in between the leader and the player making the comeback has to play mediocre at best.
Nevertheless, the biggest final-round comeback in PGA Tour history is also the biggest final-round comeback in major championship history. In 1999, Paul Lawrie started 10 shots back of Jean van de Velde entering the final round of the British Open Championship at Carnoustie. Lawrie managed to shoot an incredible final-round 67, beating van de Velde by 10, as he shot 77. Justin Leonard shot a final-round 72 to join a three-man, four-hole playoff for the Claret Jug.
Lawrie played the four-hole playoff in even-par 13, beating van de Velde and Leonard each by three shots to win his only major championship and complete the biggest final-round comeback in PGA Tour and major championship history.
The second-biggest final-round comeback in PGA Tour history belongs to Stewart Cink, who won the 2004 MCI Heritage at Harbour Town Golf Links by nine shots.