Collectors pay a premium for magnums and larger formats on the grounds that they age better. The reasoning holds, and it comes down to a straightforward ratio.
Oxygen enters through the closure
The dominant route for oxygen into a sealed bottle is the closure, and the rate depends on the cork's diameter, length and cellular structure. Glass itself admits nothing.
A magnum is sealed with a cork of similar diameter to a standard bottle, so it admits oxygen at a broadly similar rate. The neck was never scaled up with the body.
Twice the wine receiving a comparable quantity of oxygen means each measure encounters roughly half as much. Oxidative development slows accordingly, and the wine reaches each stage later.
Headspace works the same way
Every bottle contains a small volume of air above the wine, sealed in at bottling, and that oxygen is consumed in the first months.
The headspace in a magnum is not double that of a bottle, since it is determined by fill height rather than by total volume.
The same dilution effect therefore applies to the oxygen present from the start as to the oxygen arriving later.
Thermal mass adds a second advantage
A larger volume of liquid changes temperature more slowly than a smaller one when the surrounding air fluctuates. The mass buffers the swing in the same way a buried cellar buffers the seasons.
In an imperfect cellar with daily or seasonal movement, a magnum therefore experiences a smoother temperature history than a bottle sitting beside it. Short spikes barely reach the wine.
This matters more in domestic storage than in professional facilities, but very few private cellars are genuinely stable. The advantage is largest exactly where conditions are worst.
The effect reverses going smaller
Half bottles receive the same closure-driven oxygen ingress across half the volume, so they develop faster than standard bottles.
This is consistent with the observation that halves of age-worthy wine reach maturity earlier and fade sooner.
The progression across formats is continuous rather than a special property of magnums, and very large formats extend it further.
Why the premium is not purely technical
Large formats are produced in small numbers, are awkward to store and are difficult to transport safely, all of which raises cost.
They are also bought for occasions, which supports pricing beyond what the ageing advantage alone would justify.
Beyond a certain size the practical problems dominate, since a format nobody can lift or chill is a display object rather than a drinking one.