Db flies may give the effect of widening the chest because when a muscle is under a great stretch it's muscle fibers may actually lenthen towards their insertion. Would this be enough of an effect to cause a visual difference? Probably not.
Another thing you can try to do is work your pec minor in seperate from your major. The pec minor runs underneath the pec major near your armpits, and causing this muscle to grow would give the appearence of a larger upper/outer chest. To work this get into a standard bench press position and just push your shoulders forward, without moving your arms.
Other than that, nothing you can do...