I had the opportunity to try both the 38 mm and 40 mm sizes.
The 5165 felt and looked relaxed and comfortable, for that is the essence of its design. By comparison, the 5167 appeared awkward, eventhough I have a large wrist.
Big, for the sake of big, does not always improve the look and feel of a watch and IMO, the Aquanaut illustrates that very well. The new 5165 is the same size as the previous large size model, known as the "Jumbo," and that was regarded as a "big" watch.
Certainly, if one likes and is used to the current fashion for oversize cases and, irrespective of how the watch looks on the wrist, one will go for the larger case.
PP offer the cistomer a choice, "large" or "very large" and couldn't care which Aquanaut model he buys; the price is the same.