Hi John,
i think so!
And in fact, the Lange catalogue mentions a diameter of 39.5mm! So i think that´s correct.
But i´ve never measured one myself
Actually i´ve found the Dato to be slightly heavier, which to me makes a (very subjective) difference in terms of "heft".
Heavy is big, so size does matter in more ways
To me that´s been one reason the Double Split appeared outright HUGE on the wrist; it´s certainly not a small watch, but the very massive and very heavy case probably adds much to the overall impression.
But i do see your point; while the 0.5mm difference in diameter may be hard to notive at arm´s length, the 1815 CHR is 2mm thinner. And that´s something easy to see with an unweaponed eye, resulting in a seemingly larger look to me as well.
I think proportions can make a great difference.
Something subjective may be the choice of dials as well. Personally i find white dials to visually enlarge the watch, such as with the white dial 401.026 and the black dial 403.035:
(images by A. Lange & Söhne)
All in all, best to do is to try one
Happy holidays to you as well!!!!
Cheers,
Peter
(PS: a small addition - i´m trying to help out, but i´m not a moderator of the site
)