Upvote:5
The Wikipedia article on West Virginia makes it appear that Berkeley and Jefferson counties were not originally part of the state and subsequently (but in quick sequence) asked to be annexed to it, which was allowed. Whether this counts as a "voluntary" transfer from Virginia to West Virginia will depend on your perspective on the Civil War. Also, the Wikipedia article is ambiguous. You'd have to do a bit more digging to see if this was really a post-WV-statehood transfer.
Two major land changes that don't quite meet your criteria but that did have a significant impact on long-standing state borders are the Platte Purchase, which added over 3,000 square miles to Missouri 16 years after it became a state, and the reversion to Virginia in 1846 of the District of Columbia west of the Potomac...only 31 square miles, but a very important 31 square miles.
Upvote:6
Ellis Island, part of New York State, was expanded significantly over the years by fill. There was no problem with the original demarcation or surveying, but the water surrounding the island was within New Jersey, which claimed ownership of that reclaimed land. The Supreme Court eventually ruled that New York had jurisdiction to the original island and New Jersey to the fill land. The states then made a voluntary agreement to draw a borderline, i.e., to identify the territory to be ceded to New Jersey. New York had to abandon its claim to the fill, including the entire perimeter. The cession was a significant 22.8 acres.