"For many years only (4) four oceans were officially recognized, and then in the spring of 2000, the International Hydrographic Organization established the Southern Ocean, and determined its limits. Those limits include all water below 60 degrees south, and some of it, like the Arctic Ocean, is frozen.
Oceans By Size / area
1. Pacific (155,557,000 sq km)
2. Atlantic (76,762,000 sq km)
3. Indian (68,556,000 sq km)
4. Southern (20,327,000 sq km)
5. Arctic (14,056,000 sq km)"
http://www.worldatlas.com/aatlas/infopage/oceans.htm
Before the new demarcation (and in those parts of the world where the Southern Ocean is not officially recognised as a separate ocean), the sizes were different, but not the rank order of size - the Southern Ocean was previously considered as belonging to the Pacific, Indian and Atlantic Oceans. It took roughly the same area from each of the three major oceans. The reason for this new designation is that the waters of this region show significant differences from those in the more northerly regions. (It should be noted that some authorities choose to use 40 degrees, rather than 60 degrees, as the northern boundary of the Southern Ocean. This increases its size significantly, but doesn't change the rank order of the five oceans.)
https://www.nationalgeographic.com/environment/article/theres-a-new-ocean-now-can-you-name-all-five-southern-ocean
antarctica.gov.au/about-us/publications/australian-antarctic-magazine/2001-2005/issue-4-spring-2002/feature2/what-is-the-southern-ocean no longer exists
