Since 2010 Lenoble has stored their reserve wines in magnums. A number of house do this or did this, most notably Krug. The arguement is in comparison to storing the reserve wines in 225-litre barrels using the principle of the perpetual reserve or of using larger casks for instance 5000l barrels, where the ageing process slows down, the wines in a magnum with cork do not have oxidised notes and retain more freshness.
Due to climate change retaining freshness becomes more important as the acidity levels in the grapes are much lower than they used to be. Reserve wines along with adding richness and complexity should also add freshness.
Lenoble, one of my favourite houses intorduced this practice in 2010. The first Champagnes with reserve wines from Magnum coming onto the market in 2018. Yesterday we drunk a bottle of their Champagne AR Lenoble Brut Nature Dosage Zéro “mag14”. A cuvée made with 25% Chardonnay from Chouilly, 30% Pinot Noir from Bissueil and 45% Pinot Meunier from the Marne, 35% Reserve wines from Magnum from the base year 2014 and zero dosage. What was surprising was the counterpoint of richness and freshness; from the mouthfeel this did not seem like a zero dosage champagne. We drunk this with the last Vacherin Mont d'Or of the year, the 45% Meunier made this match really work.