The 270 was designed to have the same neck length as the 30/06 and that makes for a greater overall length because when you neck down you shorten the neck.
When you neck down the lower portion of the neck on the bigger calibre forms part of the shoulder on the necked down case.
If you a put a 25/06, 270 and 30/06 together then you will see that the distance from the head of each case to junction of the shoulder and neck is greatest with the 25/06 then 270 then 30/06. Thus if all three cases had the same overall length then the smaller calibre will have the shortest neck. On the other hand if you give each calibre the same neck length then the smaller calibre will have the greatest overall length.
