I believe there is a distinct difference from grapple and drag or repposition. Grapple is limiting movement in any way you can, whether with holds, bars, or pressure points. Also, grappling in real life allows using an opponents momentum and movement against them. Drag and repposition on the other hand consists of you using your own strength to maneuver the targets mass a set distance.
I picture a grapple against a larger creature being similar to many of the aforementioned plots of grabbing nosehair, hangnails, or other sensative spots to control movement as much as possible. Take the idea of grappling an airplane (that should be large enough for everyone!). By wrestling with the tail flap, a person should be able to force the plane to spin in circles until the person is removed. Similarly if a gnome was on a dragons face pulling an eyelash aggressively to the left, I would think the dragon would move to the left instinctively attempting to lesson the pressure and pain. However, a gnome picking up the dragon is completely foolish.
As a final note I support no size limits because that would make nearly every Combat Maneuver based character very very limited in end game play against such large creatures.