Because of the strength curve of the flye, the resistance is heaviest in the bottom position. With every degree your arms move up, the lever arm gets shorter and the exercise feels easier. At the top, not much tension is placed on the chest. This is suboptimal.
To get more from this exercise, use a training partner. Have him place some manual resistance against your arms starting at about the middle of the lift. You'll get a full-range challenge on every single rep and trigger more muscle growth.
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