Complex sentence: If gross tonnage is considered, the "Batillus" and "Bellamya" each with a gross tonnage of 275,276 tons, are the largest ever built, exceeding even the "Knock Nevis" (260,581 gross tons when in service as an oil tanker).
Split into simpler sentences: If gross tonnage is considered, the "Batillus" and "Bellamya" each had a gross tonnage of 275,276 tons. This is the greatest in history, including the "Knock Nevis" (260,581 gross tons when in service as an oil tanker).