Buying and cellaring wine in large bottles makes a lot of sense. Something not offered mentions, but definitely something to be aware - if a winemaker bottles wine in a large format, there is a high probability of this being a good wine, as it will represent a wine they are quietly confident and proud of. As a result, producers typically bottle a portion of their best wines into large bottles to see their evolution, and are the most sort after by collectors.
But when buying a larger sized single bottle of wine (not 2 bottles), which might suggest better value, does not necessarily always translate to being cheaper. As the materials used by the winemaker are not generally better value. As the bottle, labels and cork are made to order, magnums are not made is such volumes as 750ml wine bottles.

 

So typically the winery pays a premium for this larger glass size, and then they have to design a bespoke carton or box made for packaging and delivery. I would recommend buying magnum wines from good quality estates from around the world. Wineries which have a history or pedigree for producing wines which age well. They don’t need to be at the high price end - they can easily be at the medium end of the market because they will outlast 750ml bottles of a better quality wine and still remain affordable.
There is science behind buying and cellaring larger format wine bottles. As there is more wine in a larger bottle (e.g. magnum - 1500ml) and less ullage (the empty space between wine and closure), this means a proportionately smaller amount of air inside the bottle. Which is what causes ageing (through oxidation). A large form at wine bottle has less oxygen relative to the volume of wine, which means that the wine oxidizes more slowly. A lower ratio of air to liquid inside the bottle. Slower oxidation tends to lead to a slower and longer aging and better maturation. This in turn, positively effects aromas, and the overall stability of the wine, yielding wine that can age longer, given proper storage conditions (where the bottle in left in the dark with a constant, cool temperature and sufficient humidity to protect the integrity of the wine if under cork). The result is that these larger format bottles age at least half the speed of a 750ml bottle.
Also - larger format bottles are made with thicker, heavier glass to protect them from wine’s major enemies: heat, light and travel-related vibrations. Most large format bottle options come in dark green as opposed to clear glass, which keeps out harmful light. This thicker glass also helps with cellaring and regulating the wines temperature. As it takes longer for ambient heat or cold winter temperatures to penetrate, and either cooking or freezing the wine. Lots of vibration, like the kind you get being shipped around the world, can also damage wine, and the thick, heavy glass absorbs more shocks than thin glass that encases 750ml wines.
So there will be a prolonged maturation, and the wine will often develop greater nuances and more complex flavours than wine aged in standard bottles. A good example if Champagne - which actually benefits from being made in magnum sized bottles. As the autolysis - the gradual breakdown of yeast cells which gives Champagne its effervescence - works better in a magnum because of the greater glass surface area, which allows more contact between the yeast cells on the inside of the bottle and the wine, thus creating more bubbles. Not only can it take up to four weeks longer, this results in magnums displaying much more roundness as the wine ages and crucially, much more complexity.
Champagnes and sparkling wines will age much better and longer in magnum than most red or white wine. This is due to the carbon dioxide in sparkling wine, which acts as a preservative, helping to keep it fresher.
Because larger wine bottles can reach extremely high prices, they have earned a reputation for being expensive and described as ‘celebration’ only bottles. As wine has become more mainstream and accessible, so have magnums. You can now find more options at your favourite local winery and retailer.