To increase the number of pleats across a kilt you only need to decrease the width of the face of each pleat. military kilts are said to sometimes have pleat widths of less than 1/2"

It's quite simply really how to put more fabric in a kilt. Most civilian kilts made by bigger firms are made with economy in mind so they have standard pleat widths which are usually around 3/4" to 1" These kilts usually end up with nearer 7 yards than 8.

With a custom kiltmaker who doesn't keep a stock of fabric on hand you are more likely to end up with a real 8 Yard kilt.

Back to military pleating I think there may be a standard number of pleats put into each kilt. This number usually makes the yardage somewhere in the range of 8 Yards per finished kilt. The aprons being the only factor that will use more or less yardage. To make their standard number of pleats fit around a skinnier fellow they need only decrease the width of the face of each pleat somewhat.