Light fixture mounting bracket too small to attach to junction box
That's not a fixture box, that's a junction box (fixture boxes are typically round or octagonal). 


It looks like it's clamped in one corner so hopefully the wire is loose.

Since it looks like it's been screwed in, I would buy a metal fixture box (should fit inside the existing hole), unscrew the existing clamp, knock out a hole and reattach the clamp to the new box. If you're lucky, it should all fit in the same hole and the fixture will cover the remaining parts.

Don't forget to ground your new metal box!