A tiny amount of force can be enough to push a player over if he is off balance (say, halfway through a shot) or running at speed. Neither was the case here, but still, force seems a weird criterion to apply. I've always worked on the assumption that arbitrarily choosing to push someone over in the penalty area is a foul and therefore a foolish idea.
Such things have, for a while, made me question whether most professional referees have ever actually played football.
I know it's a multi-billion pound industry and even the tiniest of advantages can be significant, but I can honestly say that I have never pushed an opponent while playing football, or pulled a shirt.
Yet players who slightly mistime a tackle, missing the ball (not jumping in two-footed or any dangerous play) but not catching the player get automatic yellows as long as the other bloke goes down and rolls about.
The second is an attempt to play the game. The former is plain cheating.