I don't think anyone claimed that the Arc AAA is smaller than the Sonic, just that it's a nicer light. I don't have the thread link handy, but someone compared the output and battery life and the Arc AAA was brighter and lasted longer (the output of the Arc AAA was brighter over the entire life of the battery). It also has a HA-III finish (the Sonic has a softer, HA-II finish) and is waterproof (guaranteed to 50 feet, tested much deeper) while the Sonic is just "water resistent". Finally, it's worth noting that the Arc AAA has gone through several generations with a bunch of little tweaks to the design, and it is now a very reliable and stable product. I'd trust it a lot more than the relatively new Sonic.
For something I carry with me all the time and use all the time, the Arc AAA is well worth the extra few dollars.
- Russ