2 a/c identical type similar atmospheric conditions but with diff weights...
lets assume both plan a descend at idle thrust and say 250 kts.....
they both are like gliders at this point(discount the residual thrust at idle pwr).the heavier one say achieves a rod of 2500fpm. now the lighter has to increase its rod to maintain 250kts. instead if it tries to maintain 2500fpm its speed would wash off to something lower.
WHY: because a component of the wt vector cotributes towards the speed............
hope that helps