Why are Magnum bottles so expensive?

I sometimes read that a magnum of wine has fetched an astronomical price at an auction. If a magnum bottle holds the equivalent quantity of two regular bottles, why should it cost much more than twice the standard bottle price? 

Firstly, the actual bottle costs more than double the price of a 750ml bottle to produce. But the real value of a magnum (or larger bottle) is that the wine in it will mature much more slowly than the wine in smaller bottles. This is because the cork in a magnum is about the same size as that of a regular bottle, so there is relatively far less surface area in the magnum, hence less oxidation as it ages. Wine producers normally use magnums only for their very best wines and those they feel will benefit from a long maturation period. You won’t find an everyday plonk in a magnum. When you bid for a magnum at auction, you know the winemaker regards it as special and that it will probably last a long time and mature into a very special drink. Magnums are usually bought to be laid down for an important occasion. Buyers are obviously more willing to pay higher prices for something special.