The rear delts usually get neglected, but this is a pretty easy fix and one you can do at home on a daily basis. The band pull-apart will bring your sad little finger-sized rear delts up to snuff quickly.
You don't need a heavy-duty resistance band because you're going to keep the reps high. Rock bottom rep range is 20, done for 4-5 sets daily.
Arnold Schwarzenegger did a lot of things wrong. He performed endless sets of leg extensions thinking that he was "cutting up" his thighs. He believed that pullovers would spread the rib cage and pull-ups would widen the shoulder girdle by spreading the scapulae.