Original: Christ Church Cathedral is often claimed to be the smallest cathedral in England, and although it did once hold this distinction there are now smaller cathedrals, as several parish churches were elevated to cathedral status in the 20th century.
Simplified: Once this was the smallest cathedral in England. As their website says, this is no longer the case: "It is true that at one time Christ Church Oxford would have been the smallest Cathedral in England... However, with the creation this century of new cathedrals which had previously been parish churches, Christ Church lost its distinction of being the smallest Cathedral in the country.