Higher pressure pushing the same bullet to the same speed requires less powder. Leaked data for 7BC so far looks like 66grn powder to get a 170 to 3000fps, opposed to 80grn required in a 65kpsi magnum cartridge like 7 rem mag. Reduced powder to achieve the same speed with the same bullet is a shorter pressure curve and less total energy being consumed in the system (literally, powder = potential energy). So instead of wasting potential energy as heat, we get more utility from our powder on the bullet speed.

We did the same thing with the 30-06 being replaced by 308win, we can see the same thing when we load 357mag loads down to 38spcl loads, same thing comparing 45colt to acp, or 9mm para to 38spcl - oversized cases operating at lower pressures with larger powder charges have higher case to bore ratios and higher powder charge to bore capacity ratios. Shrinking cases and operating at higher pressures reduces energy smashed through the bore, and reduces how long the pressure curve has to sustain, which reduce wear in the bore.