Balance is primarily the ability to play accurately when moving at pace (crossing, shooting, dribbling etc.). It has a secondary benefit of making a player better at defending the ball as you describe, but Strength is still the largest factor in shielding the ball and not getting knocked around.
I think of Strength as weight rather than muscles; how much force will the player exert in contact with another player. Generally speaking bigger, taller players will have high Strength and low Balance, while small ones will have low Strength high Balance. But there are of course exceptions especially at the top levels of the game where elite athletes are more common.