I'm in two minds on this.
Someone has to pick it, put it into the post bag, print a label, put it somewhere for the delivery company to collect (or take it to a delivery service e.g. the Post Office), and then what happens next is covered in the delivery charge. Then when it's returned, they have to open the post bag, check the item, process the return/refund, decide whether it can be restocked (add it back onto the stock control system) and either return it to a shelf or mark it for destruction.
Usually the costs of this are calculated into the pricing of the item. If you don't keep the item, they lose this money. For every customer who does this, the company loses money.
Buying lots and returning the excess has become very common but it's problematic for small businesses who can't absorb the cost into their profit margins.
Consistent sizing would help a lot, but there are a lot of CFs out there as well who just buy loads of stuff then return it for example after posing on Instagram in it for Outfit Of the Day.
So while YANBU to want to return an item, I think we as a society are unreasonable to expect to be able to procure a product from a company and just return it and get all our money back regardless of the cost to them if the item isn't faulty. But I do think they should be clear about charges beforehand.