The term is a marketing tool, not something with take-it-to-the-bank accuracy in nomenclature. A "magnum" of champagne is 1.5 liters, compared to the standard bottle size of 750 ml. A century ago one of the British firearms makers started using the term to denote a cartridge of increased capacity compared to the standard cartridge of the same caliber, and the idea caught on. Similarly, the "tactical" and "zombie" are being used in the marketing of gun and ammunition ... heck even in the scope industries.
Of course, it doesn't make any sense at all if you don't know the standard against which you're making the comparison, but then you really don't expect a lot of precision in other marketing terms like "tastes great, less filling" either.
Here's a prime example of magnum marketing ...
I voted for my "FREEDOM", "GUNS", and "MONEY" - keep the change - UNK.
"I am always proud of my country!"
"Leadership Rule #2: Don't be an edited." - Maj Gen Burton Field.