Nations are never meant to expand forever. Eventually the ability of the state to maintain itself based on founding principals, when expanded to gigantic proportions, gets corrupted by regionalism, and factionalism and becomes impossible as expediency wins out, leading to general exploitation of one region in order to placate the one with more people or the loudest voices.
Happened to Rome, The Persian Empire, the British Empire, the USSR.
Modern travel and communication only slows the inevitable. The U.S., U.K., and Australia are unique in that we had traditions, language, and freedoms that made our nations so tight that the idea of a breakup was odious. But corruption, regionalism, and creeping tyranny always maintain the momentum to disunion. Canada is about to get hit with this because it enshrined regionalism into their laws, and exploited the west to pay off their largest city, and the French-Canadians. The U.K. is likely next, with the U.S. a bit further off if nothing changes, with Australia safe for a while.
Like my waist measurements.