unspellable:
The term 'magnum' as it relates to cartridges was coined long before the .375 H&H was introduced to the gun trade.

The .500/.450 x 3 1/4 black-powder cartridge, introduced in the 1870s, was properly referred to as the .500/.450 Magnum Express. Likewise, the .577/.500 x 3 1/8 cartridge, also from the 1870s, was known as the .577/.500 Magnum Express. They got their name from a resemblance to the magnum champagne bottle which results from necking down a large-capacity straight-walled case to accept the smaller-diameter projectile.

Yet another reason why the term 'magnum' is incorrect when applied to the .458 Winchester!