As has been mentioned, your strap is not quite wide enough to fill the gap between the lugs. I also have this model and the lug width is actually 13mm. In my understanding it would have originally come with a strap that was 13mm at the lugs and 12mm at the buckle/clasp end.
There is no doubt that the strap on the Tonneau is narrow and may to some appear feminine, but as you say, the watch itself is not particularly small and its shape (which I think is extremely elegant) necessitates a narrow strap.
You are correct to say that this model was produced in the late 1980s /early 1990s. In my understanding it was introduced in 1986.