If you make the under apron so the edge of it is running knee to hip along the thigh bone, and with a generous pleat beneath it at the right side so that if you lift your right leg the under apron is not pulled askew, then the upper apron can be made as narrow as you like, though a similarly generous under pleat would be wise, so when you sit down there is enough material to fall between the thighs.

I suspect, from the few naughtily over exposed snapshots I have accidentally chanced upon, the photographer could have been thwarted if only there had been larger pleats each side of the apron, so that lifting a knee or squatting down would simply have opened out that pleat rather than provided an unfortunate photo opportunity.

I would suggest shaping the edges of the apron, as the bottom can appear narrower than the waist due to it hanging in a curve - something on the same lines as vertical columns looking thin in the middle if they are made exactly straight sided.

Anne the Pleater