Remember how the 1920s British authority Major G. Burrard, D.S.O., R.A. (Retired), in Cartridges for Sporting Rifles, classed cartridges and used the term MAGNUM and others:

Large Bore: A rifle the caliber of which is greater than .600 in.
Medium Bore: A rifle the caliber of which is not greater than .600 in. nor less than .400 in.
Small Bore: A rifle the caliber of which is less than .400 in.
Magnum Small Bore: A small bore rifle which has a muzzle velocity of 2500 f.s. or more.
H.V. Express, Cordite Express, Cordite Rifle or H.V. Rifle: A medium or small bore rifle built specially to fire cordite or other nitro powder.
Express: Any medium or small bore rifle built to fire black powder only, which has a muzzle velocity greater than 1600 f.s. and a muzzle energy not less than 1500 ft.-lb.
Miniature: Any small bore rifle which has a muzzle energy of less than 1500 ft.-lb.

No belt is mentioned at all. Burrard rated the .375 H&H Magnum as a small bore. The Magnum moniker was used by him only regarding the 2500 fps speed borderline.