I assume you are talking about a traditional style kilt.

The aprons on a traditional style kilt will be 1/2 the waist and the hips. This means that the aprons taper and are wider at the hips than the waist but if you look at the kilt from the sides the apron edges should form a vertical line at where the side seam of a pair of pants would be.

Even if you have large thighs the aprons should still be about 1/2, and the pleats 1/2 of both the waist and hips. Large thighs are one of the things your kiltmaker should take into account when measuring.

If you are talking about a modern kilt all bets are off. Modern aprons can vary from 1/2 the waist down to 7" wide in a MUG style.