The Stanford Encyclopedia of Philosophy, under secession, notes [2]: " As the fear of forcible annexation diminishes and trade barriers fall, smaller states become feasible, and independent statehood looks more feasible for regions within states." They then continue, "Second, in roughly the same time period, the idea that there is a strong case for some form of self-government for groups presently contained within states has gained ground."
Really, the only major benefit of large states is in common military protection; another one would be facilitating economic trade, but that can be taken care of by creating economic zones, like what the EU started as, with sovereign members participating by means of standard treaties. Even military defense could be done by common consent. In Europe, quite a few countries with military ambitions were stopped by coalitions of countries.
If a region has a peaceful and prosperous culture with a defensible geography, then it tends to be more advantageous to self-govern in a small region.
With the unlimited right of secession, the main question is, if someone doesn't _want_ to be part of a country, why should they be _forced_ to be part of that country? You are born into a country, nominally without choice, but that doesn't mean that you should be required to assent to the laws. Of course, most of the time the benefits outweigh the problems, but if they don't, then it is simply coercion to make someone do something that isn't good for them. That's abusive.
[1] https://www.nationalia.info/new/10936/ten-countries-that-gra...