I know it might all be down to differences in shape, plumposity or even swagger - but I do find that a deep under apron pleat is apt to make for a cooler kilt than a narrow one.

I tend to make kilts in pairs so as to take advantage of the full length of a piece of material and not have a seam.

As I am losing weight I remake the kilts smaller and usually keep the folds of the pleats if I can, just moving them closer together. I then make the aprons smaller by increasing the depth of the under apron pleat.

Wearing a fairly lightweight kilt recently I changed it mid afternoon as I splashed one with orange juice and threw it in the sink to wash. The older incarnation of the kilt was definitely warmer than the newer one. I think the deeper pleats allow more fore and aft movement of the aprons so changing the air of the microclimate within the kilt more frequently. I even folded the aprons back on themselves so they would not cover the pleats the and it made litte difference. (My kilts are belted not buckled so this is possible.)

I suspect the the heavier the kilt the more cooling deep under apron pleats would be, when comparing the same fabric.