Many people consider the magnum, the 1.5 liter or "double bottle," to be the perfect size for Champagne, including me!
There are several reasons for this, including that many folks suspect that the actual wine placed in magnums is of higher quality for both Champagne and other types of wine. This may or may not be true for all producers, although certainly not all the wine produced in a given year by a producer is of equal quality. Might as well put the better stuff in bigger bottles.
Magnums, as well as larger bottles, are also simply more festive. Big bottles are simply impressive and fun, as well as practical for larger groups, say 4 or more people.
The most important reason is that Champagne in magnums simply ages better (and slower) and becomes more complex and nuanced than in a regular sized bottle. One theory is simply that the ratio of wine to the bottle opening and semi porous cork covering it is smaller, resulting in less oxygen contact and slower aging.
Common Internet lore is that Champagne in bottles larger than magnums is produced in magnums and then transferred into the larger bottles. This transfer process is called "transvasage" and since the wine is exposed to oxygen and there may also be a loss of pressure, champagne in bottles larger than magnums is supposedly of a lower quality.
Although transvasage does have this effect, this is not entirely true as the French Champagne laws changed in March 1998. Since then, all Champagne in bottles sized from 375ml (a half bottle) to 3.0 liters must be produced (i.e. the secondary and final fermentation must occur) in the bottle the Champagne is sold in.
Also, today most serious Champagne houses avoid transvasage for all bottles up to and including Methuselahs, the name given to 6.0 liter bottles.
And some Champagne houses, for example Drappier and Pommery, avoid transvasage for all Champagne regardless of bottle size.
So theoretically, Champagne since 1998 in bottles larger than magnums should be better - age more slowly, developing more complexity and nuances - simply taste better than magnums. I can't tell you if this is true from experience, and no one else can definitively either.
I can tell you magnums of Champagne rock. They're fun, they taste better, they last longer, and they're just a perfect size. Ask me in a decade or two about bigger bottles. I plan on personal research!