I think part of the problem is that you've got the small knight directly in front of the troll, and then the giant warriors directly behind him. Because they're both human shaped your brain assumes they should be the same size so it spoils the illusion of depth you've created with the value shift.
Detailing the ground beneath the fight will help, and maybe add some 'human scale' figures alongside the giants in the background, to help maintain a constant sense of scale throughout the piece.
Don't get me wrong though, its still spectacular!