Superbalist - not superb at all!

Superbalist is refusing to refund/exchange a pair of short pants I bought from them, which cost me R299.00, even though it is far too small for me. Now just how am I supposed to know that the size Large from one supplier - Style Republic - is significantly smaller that the Large size from another supplier - Cotton On, from which I have already bought 3 pairs??? There are no indications that this is the fact. So now they are forcing me to keep clothes I cannot wear, thus I'm losing R300? This is exactly why I very rarely shop online and now because of this experience it's going to be a long while before I buy from Superbalist again, if ever.