Problem is you are comparing the best of one (heavy, high BC bullet), to the absolute wost of the other (light, low BC projectile). With the right loads you can make the 7.62x39mm seemingly outperform the .300WM...or the .25-20WCF shoot flatter than a .25-06Rem.As for the wind the 25-06 has no more wind buck than a 300 win mag comparing as close to similar weights as possible.
Now we arrive at the root of the problem. The 6.5mm & 7mm affords more bullet selection than anything else. 6mm & .30cal. are close runners up. OTOH the .25cal. affords a pretty poor selection, but even still there are a few gems (like the one I listed in the ballistics comparison)....but without ever haveing loaded for either I have to go off factory ballistic data.
I do most of my business with MidwayUSA. The key isn't the number of different bullets (though there are a few more in total; with 5 pg. of 6.5mm and 4 of .257cal.), but the selection of weights, profiles, and types of bullets. The 6.5mm excels as you can get everything for it, most importantly bullets with a BC exceeding .600 which is pretty darn efficient!Where are you finding the selection for the 6.5mm (260) because I normally get my stuff from Natchez or Midway USA and it seems as though the bullet selection from Berger, Sierra, Speer, Hornady and the others is about twice as much in .257 over 6.5mm. The .243 and .30 cals are alot more abundant in choices as the rest.