I guess the extra 50mm in the 26700s makes a little difference.

Ummmm, I don't know how well acquainted with the metric system y'all are, but 50mm would make A HELL OF A DIFFERENCE. For every battery, the first two digits designate the diameter of the battery in millimeters (e.g. 18 in 18650, etc.), the second two the length of the battery in millimeters (e.g. 65 in 26650, etc.) and the last digit is not actually a digit but a symbol for the shape of the battery (0 meaning the battery is round).
So a 350 battery is not 350 mm long, but 35 mm long and of round shape. The same goes for the 500s and 650s.