My understanding is that the thickness comes from the fact that the body isn't a single piece of metal, so you can swap out the RAM and HDD (unlike the 31A and 21A). Also, apparently you can repartition the SSD that's used as an OS cache into a separate drive reasonably easily.

As much as I'd prefer the formfactor of the 21A, the ability to upgrade to 8GB of RAM is too big of a selling point to pass up on the 32VD.