I wasnt sure if this was the right place to ask the question, but thought Id give it a go. I guess Im really trying to understand if larger nations have a propensity to fragment after a long enough interval and for any reason. Examples being old and modern empires, USSR, etc. On the other hand, what is the frequency of smaller nations banding together to form a larger nation like Switzerland, USA, UAE, etc.
Thanks!
Edit: Thanks again for the replies
I personally haven't read anything but that doesn't seem to make sense because it puts more emphasis on geographical size rather than human agency.
Let's take France for example. Roughly, France has existed with it's current geographical borders for almost two hundred yeas. It expanded out of interest of Louis XIV and his personal interest to expand France to it's natural borders; the Rhine, Alps, and Pyrenees. It would end up stabilizing into the shape it is around 1815 when the Allied powers decided to limit France to it's borders as of 1789. Beyond that, France had only lost territory during the Franco-Prussian War in 1870 and gained it back after WW1.
Meanwhile, you have Germany only being founded in 1870 after the defeat of France and then partly dismantled with Poland being made after WW1 and being more dismantled after WW2 into East and West, only having being unified in 1989/90. So Germany went from being a very large power before WW1, still having lots of land after that, then being destroyed and split into two after WW2. The reason? The Iron Curtain and the aggressive push of Russia/USSR to spread Communism.
So, it isn't so much a matter of size but of human factors.
I don't think there are many studies into this, but in general it's not so much the size of the country as much as the cultural groups that exist within it- and the neighbours surround it that determine whether the country will stick together.
If there are many cultural groups with more than one distinct religions, languages, cultures, etc. (see nations) there is a high likelihood that at some point that nation will try to separate (see Quebec, Kurds, Serbians) If that group is more similar to its neighbours, it causes even more problems because it's neighbour will probably support its separation (Ukraine & Russia recently)
Aside from that, the neighbours of a country also determine whether a country stays together based on how much it is susceptibility to invasion. The island of Britain, once united with Scotland has pretty well stood strong against all invasion attempts from many different countries. Mostly because it's an island. Whereas countries like France have had their borders moved and their territories chopped up and re-allocated so many times throughout it's history it's crazy!