The red channel is shifted away from the blue and green channels. Your eyes are no longer receiving the full force of the brightness of the original image. One eye gets only the red channel and the other eye gets only the blue/green channels. And as far as I know, you don't add the brightness received in one eye to the brightness received in the other eye to get the total brightness. You get 20%-30% deduction in brightness.
The red shift is suppose to be there. That's what makes it 3D. But to brighten the overall video, you could add a Color Curves effect to the Output FX. There's a preset named "Brighten Brights" you could try. I sometimes the Color Curves effect and use the "Increase Contrast" preset and then tweak it. Of course doing all these extra corrections could reduce the 3D effect and cause ghosts.