This dress model farshi salwar is supposed to be worn in loose/oversized fit. As per the agent from jonaya support team the model is S and is wearing a M size dress to make it appear as loose fit dress. It was not mentioned anywhere on the website that the customer is supposed to take one size above the usual size or provided the info that model is wearing one size above. There is no extra fabric as well to make adjustments.
The website is useless as it does not have any proper info and customer support is of no use because they cannot help customers in exchange of a size even when the mistake is from their end.