I would guess that it had more to do with the powders available at that time than the length of the action. For example, I owned a Rem 700 in 8mm Rem Mag in the mid-1980s. Performance was so-so with 220 gr bullets and H450 (3050 fps, loaded HOT), but that was about the slowest powder available at the time. With my 375 H&H loaded with IMR4350, you could shake the case and hear the powder rattle around on the inside, but I was almost at max load. Nowadays we have H1000, Retumbo, IMR7828, BMG50, and a bunch of other powders that are best suited at launching very large bullets from very large cases. I have noticed a few of the full-length magnums are picking up several hundred fps over their max loads of 20 years ago, due mostly to the newer powders.