Everywher I look it says VOS is the biggest ship ever, but it's stats (size, passenger space) are the same as it's sister ships. Are they all the same or is VOS bigger?
When you read that Voyager is the largest cruise ship ever, you are reading old copy. As the first of her class, she really was at the time the biggest ever.
On some lists I have seen, VOS is smallest of the five (in terms of gross tonnage, which is a measure of volume, not weight), but we are talking small differences among the five.
The tonnage is a measure of space. 1 GRT = 100 cubic feet of accessible air space. The the newer ships have made better use of space within the same hull design.