Here is a JPG image (unprocessed) followed by the corresponding DNG that I post-processed in Photivo/GIMP. (Canon Powershot SD750)
Flip between the two images in Chrome. Why is the raw image slightly larger as well as softer?
Of course I don't know. :)
However the most common causes for these things are:
In your concrete case, I think that at least the first 4 points above apply.
Yes, known fact. And more: it differs depending on what RAW conversion engine you use. See here:
It has to do mostly with how the demosaicing engine treats the near-edge R, G or B pixels in order to compose the actual picture.
For reference, Photivo uses DCRaw.
Also, some JPEG engines truncate the dimensions of the image at multiples of 8, other ones do not.
So it isn't a surprise that the dimensions are different. I know, it doesn't feel natural, but this is the way it is. Perhaps when we'll get rid of the classical single color (Bayer/TransX etc) sensors we'll get rid of these oddities as well.