The compact versions are meant to be more portable and overall less bulky. This means shorter barrels and smaller bottles. All the original Prophet Compact have 400mm (16”) barrels, while the Prophet II Compacts have 500mm (20”) barrels (except the 4,5mm/.177 which has a 510mm barrel). Instead of the standard 320cc aluminum bottle, the Compact versions have a 240cc bottle, which makes them much less bulky. Energy wise there is, of course, a compromise as shorter barrels mean slightly less velocity.