Cologne Cathedral is a gigantic brooding monolith. The reason why it is so dark is not due to a lack of maintenance (few cathedrals spend as much time with scaffolding as does this one), but because it is made of porous sandstone... a soft rock that unfortunately absorbs air impurities rather easily. Most of France's famous cathedrals are made of Limestone, which is better protected from weathering.

Besides being on such a massive scale, the true glories of the Cathedral are its' stained glass windows, and above all its' treasures. The most famous is of course the Shrine of the 3 Magi. But the 1000 year old monumental Gero Cross is worth a visit, as are the cathedrals many items in the treasury.

Cathedrals are measured by floorspace... so yes Seville and Milan have more of that. But the 150 ft. high vaults and 515 ft. tall twin towers, more than make up for that. In sheer mass, Cologne is second only to St. Peters in Rome.