A jeroboam or double magnum is the size of four bottles or 3 litres, except in Bordeaux where a jeroboam is five litres It’s a real centerpiece for the table.
The larger bottle means the wine ages more gracefully in the cellar, as the air to wine ratio in the bottle is smaller than for the 75cl bottle. The cost of production and rarity of these bottles means that they cost a little more than their bottle equivalent.