Chris has it nailed perfectly on this one. I have both colors and it is the back panel. Everything else is identical, the band and front panels are the same thickness. Its for sure the back panel, but as to why Chris might be right there also but I don't know.
I doubt I'd go that far with speculation. It seems that any/all thickness increase comes from the back panel only. In almost every pic, that's the only part that seems increased in size...the other points such as the antenna band usually line up perfectly. So I'd say it's probably due to the added thickness Apple had to make it (the back panel) to keep the LED flash from bleeding through during photos. I have an aftermarket white, and also a yellow backplate, and when the flash is on, almost the entire back panel lights up because of bleed through.