You don’t consider 80k a significant step up from 20k?
Oki was an 82 day battle, Iwo was 35 days. Oki had a monster sea battle against the kamikazes, Iwo didn’t. At Iwo, the American casualties surpassed the number of the enemy forces. Iwo Jima US casualties were 26000. As for a the significant step up in casualties, the naval forces suffered more casualties than the ground forces, definitely not the case at Iwo Jima.