I think that's because the body is instantly at maximum velocity (because of the very high gravity). Box2D has a maximum velocity and I think it's 2m per step. It seems that you run it on a mobile device which are usually limited to 60FPS. That means that you have max 60 update steps per second, resulting in a maximum velocity/speed of 120m/s. By the way, your update rate is framerate dependent.
I see that you have defined the meter-to-pixel and pixel-to-meter constants, but you aren't using them anywhere in your code. That means that you are using a meter-to-pixel ratio of 1:1 and thus the maximum speed any body can have is 120px/s. That's not a lot and judging by that video it could be correct.
A common trick is to scale your camera's viewport down instead of scaling all box2d related things up. Like this:
camera.viewportHeight = Gdx.graphics.getHeight() * WORLD_TO_BOX;
camera.viewportWidth = Gdx.graphics.getWidth() * WORLD_TO_BOX;