If I remember my basic aerodynamics correctly it goes something like this...
Ground effect is essentially the reduction of induced drag (the force that wants to slow you down) due to the ground interference with the formation of wingtip vortices. Since you essentially reduce the total drag on the airplane, the forces of thrust, and to some degree lift, are experienced to a greater degree. That's why the airplane "floats" as it nears the ground.
Ground effect is begins to be experienced within approximately 1/2 the wingspan of the ground, that is to say if the wingspan is 100 feet, the ground effect begins at about 50 feet.
Since air carrier aircraft have such large, efficient airfoils, they feel ground effect to a greater degree than do many GA airplanes.