That's not true at all. Maybe for the given shape of the pixels on these displays, but as a general statement, this is false. If you have red, green, and blue

That's not true at all. Maybe for the given shape of the pixels on these displays, but as a general statement, this is false. If you have red, green, and blue subpixels that have exactly a 3:1 height-to-width ratio -- thus forming a perfect 1:1 square -- then you could have zero black space on all sides, and the pixels would appear perfectly square.