It's a simple concept. All barrel lengths chosen are (like most things) a compromise between weight, handiness and technical issues. In the case of barrel lengh, standard cartridges don't need such a long barrel as magnums....as they carry less powder. A longer barrel for the magnum loads allows more of the (large) powder charge to be burnt...thus propelling the bullet to the higher velocities for which magnum loads are designed. With this in mind, it is almost always true that a magnum cartridge will suffer a greater loss of velocity from a shortened barrel...than will a standard cartridge. For example, if you compared a .308 Win.....and a .300 Win. Magnum....the .308 would usually get a 22" barrel (actually, a 24" barrel might be better....but 22" is a compromise). The .300 Win Mag needs at least a 24" barrel (even 26" to 28" would be better...again, it's a compromise), to extract all of the performance of which it is capable. If you were to cut the .308 barrel to 20"....and the .300 Win. Mag barrel to the same length.... the Magnum round would lose much more performance (over that of a more standard barrel lengh)....than would the .308. This is why most magnum rifles come with at least a 24" barrel....and often, a 26" one. Many custom magnum rifles come with barrels even longer than 26".

By the same concept, there is a maximum length, above which additional velocity is not worth the extra weight and unwieldyness. For standard rifle cartridges, anything longer than about 24" offers little advantage (and indeed, most ammo companies use 24" test barrels for their standard cartridges). For magnum rounds, often 26" is the standard test barrel length....but, not much would be gained from going beyond 28"...

I hope that this explanation will help clear the mystery.