3-1/4" Pancake Box? - Want to Update Old Ceiling Fixture
Many things don't fit 3-1/4" boxes

There are quite a few fixtures and other pieces of hardware that simply do not fit a 3-1/4" round or octagon box, as their mounting holes are too far apart.  This can be sometimes fixed for fixtures with the aid of a "universal crossbar" that adapts the mounting holes to the right places, but in some more exotic situations (like where excess box fill or the nature of what's being hung off the box forces you to put a mudring or extension box on for volume or mounting space), the requisite parts simply aren't made.