Question: Much later, West Virginia broke off from Virginia as a result of the Civil War. Earlier than that, Maine became "independent" from Massachusetts as part of the Missouri Compromise. 

But during and after the Revolution, Virginia claimed Kentucky as its "backland." In 1795, it relinquished this claim to let Kentucky split off to become the 15th state. (Vermont had just done that from New York to become the 14th). Why was that?

Apparently, there was pressure from the other states. Was it because they wanted all the states to be about the same size in area and population, or because they wanted to prevent an "arms race" of superstates like Virginia-Kentucky, North Carolina-Tennessee, Pennsylvania-Ohio (in today's terms) etc.?

Answer: 
Prior to the signing of the Constitution, most of the 13 colonies [ceded all territorial claims](https://en.wikipedia.org/wiki/State_cessions) to the Congressional Government in exchange for it assuming their debt. Virginia followed suit 3 years later. This made the Kentucky Territory under Federal jurisdiction, and later it was organized into another state.

The text of North Carolina's Act of Cession: [HERE](http://www.constitution.org/uslaw/acceptance_cession_north_carolina_western_territory_1790.html)
For a corollary explanation to [Oldcat's answer](https://history.stackexchange.com/questions/26912/why-did-virginia-let-kentucky-become-a-separate-state#answer-26919), try reading [The Division of Territory in Society](http://www.edstephan.org/Book/contents.html) by Ed Stephan. It's a very interesting read, even if you aren't a sociology student. He goes into quite a bit of depth into why counties kept splitting and why they have stopped splitting in modern times.

The American people took their roles and responsibilities as citizens more seriously in the 18th and 19th centuries. If it took too long for them to make it to the county seat and participate in the operations of the county, they would complain and petition for the county to be split. When the county split, the seat would be chosen near the center of mass for the population. You can see the procession of county-splitting in Kentucky with [this interactive map](http://homepages.rootsweb.ancestry.com/~george/countyformations/kentuckyformationmaps.html).

As Oldcat mentioned, Virginia, North Carolina, South Carolina, and Georgia all had territory extending clear to the Mississippi River. The people in what became Kentucky, Tennessee, Alabama, and Mississippi couldn't hope to make it to the state capital (Richmond, Raleigh, Charleston, or Atlanta) in a reasonable period of time, especially with the Appalachians in the way, so they told their senators and representatives to split up the state.