Draping the chiton

[Giannis K. Hoplite]

Pleating in linen can be achieved by tightly twisting the fabric into a knot while wet and let it dry over heat in that position for almost a couple of days depending on the environment.

Pleating in wool can be achieved and is also backed by archaeological finds, by twisting the threads of the warp and weave under tension.

Some chitons might have had pleats sewn on the shoulders, but this will not make the garment less bulky at the waist.

Finally, artistic representations very often show very bulky garments, especially in the periods where realistic depictions were more popular. Archaic depictions for instance tend to show very tight garments, so tight that they could only work if they were modern elastic.
Late classical depictions wanted to be both realistic and show the beautiful human body. They compromised the two by showing figures in action and their garments conveniently blown against their bodies and flowing behind them. This is an illusion though, making the chitons look narrow. Look at the Nereid monument as an example of this.

And if the length of the fabric needs to be A to accommodate neck and arm holes, it can't be <A at the rest of the garment.

As a result, the answer to your problem is correct materials.