Beretta currently makes three frame sizes for the 680 guns: 12-gauge, 20-gauge, and the "baby frame" (28-gauge and .410). According to an older Michael McIntosh gun review, the Beretta baby frame 680 28-gauge is 7/64-inch shallower than a standard 20-gauge 680 frame and 1/32-inch narrower at the knuckle. The width of Beretta 680 frame is largely dictated by the amount of steel required around the locking pins. Thus, there is relatively little difference in overall width. However, the side-mounted locking pins allow the Beretta to be one of the shallowest frames available per gauge.
You can get 28-gauge barrels on any 680 series gun but the baby frame guns are available on the 687 Silver Pigeon III and higher grades. Keep in mind that Beretta seems to change their model names and configurations on a monthly basis.
