The elastics that we use to design the scrunchies hold both thick and thin hair perfectly. This is why the only difference between the two available sizes is visual, the elastic inside is the same.
In the photo below, you can see the difference in size between the "small" on the left (around 9cm in diameter) and the "classic" on the right (around 13cm in diameter):
Two burgundy scrunchies (hair accessories) next to each other