Okay, let's see if I got a grasp of thread count and sett size. I am going to use the tartan I have selected for my first kilt, MacLeod Red wr496.
Thread count goes like this;

B8 R2 B2 R4 B22 R4 B2 R2 Y2 R2 B2 R32 B16 R8 G8 R32 G22 R16 B8 R4 Y4

Now the sett size for the above would be determined by the thickness of the individual thread times the number of total threads in the count, in this case 202.

Now for the sake of guestimating my sett size I shall make the assumption that a single thread is somewhere around 1mm in thickness. This results in a sett of 202mm give or take dependant upon the tightness of the weave. 20cm is roughly 2/3 of the imperial foot so roughly eight inches more or less.

Am I correct so far?

Now on to enlarging or reducing the size of the sett. I see two possible approaches, one workable and one not.
First one can increase the individual thread thickness which is actually an increase in thread diameter and weight. It seems this is a case of diminishing returns because as thread diameter goes up so does the per yard weight of the cloth. Your 16oz tank suddenly becomes a 32oz or greater with only fractional increases in sett width. To reduce the sett by reducing thread thickness has the opposite effect of reducing 16oz cloth to 8oz cloth. Not a workable solution for kilt material since our desired material weight is a fixed variable in this equation.
The much simpler method would be the increase the actual width of each stripe within the sett and leave the threads alone. For instance to double the size of the above tartan above the weaver would just set up the following thread count.

B16 R4 B4 R8 B44 R8 B4 R4 Y4 R4 B4 R64 B32 R16 G16 R64 G44 R32 B16 R8 Y8

With the same 1mm thread size we get exactly double sett width of 404mm or roughly 16 inches. Of course by not thickening the thread the per yard weight of the cloth remains the same. As long as the original thread count is multiplied or divided by the same integer for each and every color the pattern will remain the same in all areas save proportion.

So do I have my mind wrapped around the solution?