I agree with M and others that it's about the USD right now, but the more important question going forward is how long that will last, and how it will end. If it gets too big it will either get crushed under its own weight and everything will languish, or it will fly apart, scattering in many directions.
How it will actually happen depends on its structural and dynamic characteristics, and on what type of infrastructure is in existence to provide exits.
My bet is that it will be neither crushed nor fly apart, but that it will instead crumble. It will be too big and homogenous to provide any good room to attach to firmly at its perimeter, and those loosely held at that perimeter will have a tough time hanging on, and will fall to the ground--crumbling, as it were. There will still be a solid core, with a crumbly mantle.
The crumbs will fall and aggregate on the ground, until they themselves begin to provide viable options for agglomeration, as opposed to the USD.