Flys are further proof of the value of isolation movements for size.
What Arnold's doing and anyone should try on both flys and presses is emphasizing the bottom 2/3 partial ROM, which is the sweet spot for chest. That's for several reasons:
-Any free weight exercise that uses an arc like flys or curls, only works the muscle through part of the motion; tension's lost outside of that range - generally the top and bottom portions of ROM. You can avoid those areas and work the muscle better, IMO; there is a sweet spot for any exercise, where you can use more weight and the muscles have more leverage. For chest it is close to the beginning/middle of the motion, while the end portion of ROM is more for tris. Therefore work the lower/mid portion of ROM for flys and presses.
-Cheats and partials are ways to emphasize those areas where the muscles have a mechanical advantage and can move more weight, and are under tension. Generally those areas of ROM work the muscle more effectively. That's why cheat curls can be better than strict curls, why it's not necessary to do the bottom or top of preacher curls, and why box squats can be better than full squats for size.
Also it's true that dumbbells can be more effective in isolating the chest AND are less harsh on the joints. In addition, the use of dumbbells allows the hands to be at different angles. Try dumbbell flys using a neutral/palms facing one another grip, or any angle between neutral and the usual grip.
