When you're in the missionary position, have you tried it with your legs up/ankles or feet by his shoulders? When we're doing missionary having my legs up helps a lot. My favorite position is being on top with my husband sitting up.
I'm a plus lady! And best positions I've found are missionary, cowgirl, reverse cowgirl, laying on my side with one leg in the air (also gives the man some extra grip), even on the edge of the bed on my back and him kneeling/standing on the floor or leaning over me on the bed slightly.