Seems self explanatory, but to propel the bullet fast enough to really justify the extra weight (and without using longer barrel lengths, or seating the bullets to a a longer OAL in a long action receiver).
Look at the ballistics, and compare the Sierra MK's in 150 (0.429 BC) & 168gr (0.490) bullets. With a MV of 2800fps, the 150's are 1515 fps at 1400 yards with 12.2 mils of drop. The 168's, at 2700fps, are 1575 fps at the same distance with 12.4 mils of drop. Even stepping up to the 168gr Nosler CC bullets, which publish a BC of .520, the velocity is 1631 fps with 12 mils drop. Between the three, drop and velocity is again virtually indistinguishable at 500 yards. My long range "fun" rifle is a 22" 7mm-08, and my initial thought process was to run the 162/168s for 600-1k yards...but outside of the cheaper price of Nosler 2nds, there is no reason in my short action 7mm-08 to shoot the heavier bullets (unless shooting subsonics suppressed).
Of course, all of this is irrelevant with regards to hunting bullets. Hunters have been using the 7x57 for years with heavies at slower velocities for damn near a century, killing virtually every game animal on the planet.
The heavier bullet catches up in velocity well before either has dropped to a level where bullet expansion is questionable (sub-500 yards, depending on the specific bullets and launch speeds), which means that the light bullet will reach minimum expansion velocity at a lesser distance. Additionally, there's not enough difference in elevation between the two bullets to matter (0.1-0.3 MRAD difference), since you'll be dialing for either bullet beyond the PBR (which only differs by a handful of yards, again depending on specific bullets and launch speeds), and beyond about 600 meters the heavier bullet drops less than the lighter bullet. Finally, the heavier bullet has less wind drift right from the muzzle and only continues to drift less and less as distance increases. So how exactly can the 7-08 not propel the heavier bullet fast enough to justify the extra weight? I see plenty of justification where it counts.
