Lug-to-Lug
Lug-to-lug is the vertical distance from the tip of the top lug to the tip of the bottom lug. It, not the case diameter, decides whether a watch will actually fit your wrist.
Most people check the case diameter, but comfort is governed by the lug-to-lug. If that distance overhangs the flat part of your wrist, the lugs dig into your skin no matter how thin the watch is. A practical rule: lug-to-lug should not exceed the width of your wrist. Slim wrists are usually comfortable up to around 44 to 48 mm lug-to-lug, which is why a 39 to 40 mm watch often sits better than a 42 mm one.