Over-sized rackets: + Larger sweetspot, so more forgiving of off-centre hits (may also generate more power for any given swing speed, due to the longer strings). - Greater aerodynamic drag, so swings much slower (a player with good technique will generate more power from an iso or oval frame). Essentially, they're good for beginners/intermediates or more defensive type players. Oval frames are pretty much the exact opposite - less drag than iso frames, but more forgiving.
oversize head also has slower response. you will need to string it tighter to get the same speed out of the strings.