Wow, I've never seen such a big game universe!
Yeah, the physics behavior has a finite world size. The Box2D docs don't make it clear the purpose of the world bounding box - it's just been made as large as reasonably would be expected to be useful, which does seem to end about 20,000 by 20,000. It vaguely mentions it might improve performance to use a smaller bounding box, but I don't know why. What you want is effectively an unlimited physics world size anyway - I'm not sure that's possible with existing physics engines.
You might be able to get some mileage by changing the world x/y scale (either make it really huge, or really small, I forget which). Apart from that, I think you're pushing beyond the limits of a physics engine. I don't think they're designed to span billions of pixels.