Very good point, Richard. This could be a factor, as early nineteenth century sett sizes were smaller.

I cannot seem to locate my copy of Bob Martin's All About Your Kilt at the moment, but in there he documents the details of the oldest known tailored kilt, which is a Gordon Highlander's kilt c. 1796. It contains just barely more than 3 yards of cloth, and is box pleated to the stripe. But the number of pleats is fairly high. I don't recall precisely how many pleats, but it was in the neighborhood of 18 or 20 if I recall. But the sett size of the tartan was also extremely small, compared to how the Gordon tartan is woven today.

Of course one could also argue that an equally valid response to an increase in sett size would have been to simply make the kilt with fewer pleats - and indeed we have examples of early box pleated kilts which did just that. (Oh, where is my copy of Bob's book!?)

This also raises the question, why did the average sett size increase? Most likely simply a change in aesthetics.