I'm not sure that's true, I'd assume gravity is doing _some_ kind of job in keeping them inside by pulling down. I think losing them would be a lot easier if they were very light.
In the sudden movement scenario, gravity is overcome by rapid acceleration in the upward direction. In the no-dropped-item scenario, friction within your pocket (presumably) comes to the rescue to make up the difference between gravity and upward acceleration so that the item doesn't make a successful escape.
The friction in a pocket should be a function of tension (how tight your pants are, especially around the item) and not, as often is the case, proportional to the mass of the item. So this particular method of losing the item should be less common with something lighter.
