One more time. You don't shoot a bow. You don't shoot an arrow. You don't shoot a broadhead. You shoot a system consisting of a bow, an arrow and a broadhead. It is your responsibility to make sure all of those items support each other.
Example 1: 70 lb bow, large expandable and sub 380 gr arrow, expect failures.
Example 2: 70 lb bow, cut on contact 1 1/4 inch cut or less, and 350 gr arrow, probably going to get through just about any deer as long as you do your part and don't hit heavy bone.
Example 3: 70 lb bow 500 gr arrow, large expandable, probably going to get through just about any deer as long as you do your part and don't hit heavy bone.
Example 4: 70 lb bow, cut on contact 1 1/4 inch cut or less, and 700 gr arrow, probably going to get through just about any deer regardless of where you hit.
You should have no problems going with the lighter arrows as long as you are not using a large expandable/mechanical head.