One aspect of irons involves the width of the heads.
A few years back, some irons came out with Oversized head versions. These include the Ping i3 O-size (2001), PowerBilt TPS 6.0 (1999), TaylorMade RAC.OS (2003), and Titleist DCI Os (1995) and 804 OS (2004).
Oversize heads were longer from toe to heel (a wider face) than standard irons of the time. The wider a clubface, the larger the C dimension (a measurement designed by Ralph Maltby of the "horizontal distance from the clubhead's center of gravity to the intersection point of the the hosel centerline with the groundline.")
For details on the above drawing, see http://www.ralphmaltby.com/108
Since then, the term Oversize has fallen by the wayside, and the larger heads tend to be in the SGI irons (Golf Digest categories of Player's, GI, and SGI). Examples include the Callaway X20 and the Ping G10. GI clubs would include the X-family Tour models, and the i-family irons. These have narrower clubfaces.
It's all about tradeoffs. Larger clubfaces have more face surface to put onto the ball, whereas smaller clubfaces are less likely to hang up in the rough, and also have better feel (less metal to dull the tuning fork effect, so to speak).
The SGI irons also tend to have thicker toplines (when looking down at top of iron's head when addressing the ball) which certain golfers find irritating. This includes some of our ST posters.