There are fixed overheads involved in processing a transaction. A small transaction has almost the same overhead as a large transaction. Thus it often makes sense to give a better rate on larger transactions. This is especially true for traders who themselves buy in bulk to get lower fees up the chain. If you do a big sale, that may be enough for you to do a large purchase and book a profit immediately. If not, you may have to sit on the currency you got paid until you can justify a trade yourself, and that means you take more risk.