As I might have already revealed, I am slightly (maybe 17.48%...) obsessional when it comes to the measurements of the watches.
Today, I borrowed some digital calipers and found that the watch I was wearing (a G8000) measures 41.71 x 41.84 x 14.02, whereas Casio gives the measurements 52.5 x 40.4 x 11.2.
This might be a slightly extreme example, because Casio would seem to measure height from band-lug to band-lug, and on the G-8000, the band-lugs are visually part of the band, not the case. On the other hand, they don't measure the buttons when measuring the width.
The depth is quite different though. Casio seem not to include the caseback, which I think most of us would be inclined to include. Also, in the case of the G-8000, they don't measure to the highest point on the face of the watch, which is the flashing light housing.
I must do some more measuring!