Omega market Speedmaster as a 42mm watch, which when you measure on the case diagonally is actually correct and gives you the exact 42mm dimension too. This is primarily because of the lip/bevel on the crown side of the case which adds 2mm to the watch.
If you compare this with Rolex, or let’s be more specific,, if you compare it with the Daytona, and apply the same measurement as Rolex do then it is a 30mm watch i.e. Measuring the case at the outer extremity of the bezel gives the reading of a little under 40mm.
It is worth noting here that the face of the watch is typically what makes up for the substance of the watch as it looks on the wrist, while lug to lug is what defines its wearability.
See the photos bellow to see how the face of the speedy is actually only 40mm making it more like a 40mm watch as opposed to it’s marketed size of 42mm.
Let me know what you think about it and share any examples if you have, where the marketed size of the watch is a little deceiving.