Two watches with the same diameter can fit very differently. Lug-to-lug explains much of the difference.
Watch diameter measures across the case. Lug-to-lug measures length: from the tip of the upper lugs to the tip of the lower lugs. Long, straight lugs make a watch occupy more of your wrist; short or sharply curved lugs keep the case more compact.
The practical check is simple: the lugs should not visibly overhang the flat top of your wrist. A close fit can still work when the lugs curve downward and the strap drops cleanly, so the number is a guide rather than a verdict.
Bracelets with a rigid first link can extend the effective span beyond the published case measurement. If possible, check a wrist photo or try the watch on with the actual bracelet.
Watcher Insight: when comparing watches, read diameter and lug-to-lug together. Diameter suggests visual presence; lug-to-lug is often the better warning for overhang.
