Would it be possible for you to show us a photo from the rear so we can determine if the kilt is the wrong size or perhaps just being worn wrong.
One of the most common causes for the shower curtain folds may not be an incorrect size but a kilt that is designed to be worn right up, under the ribs, that is being worn lower.
For example - This kilt is the correct size, it is just being worn in a way other than how it was designed to be worn.
The bottom of the sewn down and tapered part is called the Fell Area and the bottom of the Fell Area should be right at the widest part of the hips and buttocks.

(Notice please that this kilt, when worn like this, has popped some stitches. When sitting the Fell Area goes under the butt and the stitching has failed due to the stress)
Steve Ashton
www.freedomkilts.com
Skype (webcam enabled) thewizardofbc
I wear the kilt because: Swish + Swagger = Swoon.
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