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)
Kentucky was admitted on June 1, 1792, not 1795. It was a part of Virginia until it became a separate state. In that respect, it differed from Tennessee, which was a part of North Carolina that was ceded by the legislature to federal jurisdiction, and was an organized incorporated territory of the United states, called the Southwest Territory, for five years before becoming the state of Tennessee.

Virginia's legislature consented to Kentucky's admission as a separate state as early as 1788, when the Articles of Confederation were still in effect. Congress's deliberations on whether to admit Kentucky were interrupted by a notification that New Hampshire had become the ninth state to ratify the new Constitution, which then went into effect in the ratifying states. They decided to defer the matter until the new Congress under the Constitution could take it up. Virginia's legislature reiterated its consent in 1789. The act of Congress admitting Kentucky takes note of the consent of the legislature of Virginia. It was passed by Congress on February 4, 1791, two weeks *before* the act admitting Vermont was passed, but it said the new state was not to be admitted until nearly a year and four months later, because Kentucky politicians had asked for some time to get things ready. (Vermont, on the other hand, was admitted only two weeks after the act of Congress was passed and signed.)

I think the expense of governing so vast a territory under 18th-century conditions may have played a role, and also a desire to increase southern representation in the Senate (or ealier, in the unicameral Congress of the Confederation).

Here's an article that explains some of the history of Kentucky's admission as a separate state: [When Did the Articles of Confederation Cease to Be Law](http://scholarship.law.nd.edu/ndlr/vol78/iss1/3/).