This YouTube video, posted by prolific mapping channel EarthDirect, shows the evolution of boundaries in what we now call the United States of America from 1629 to the current time.
The boundaries first highlight European colonies, then states and territories, as well as county borders within them along the way as applicable.
What we think of today as Maine lacked its top half until the 1760s, and was of course a part of Massachusetts until 1820.
This video illustration shows Maine settling on its current shape right around 1764 (although the counties change form quite interestingly for some years thereafter).
In that regard, it doesn’t take into account all the dispute around the northern border of Maine in the early 19th century, during which time the U.S. and United Kingdom were heatedly trying to decide where this state would end and the British colony of Canada would begin.
But because the exact border was in dispute during that time, and could have been drawn in a number of places, depending on whose side you were on and which old documents you read, it was probably easier for EarthDirect to just use today’s northern boundary and call it good.