Why would it force the differential down? If anything, a greater proportion of smaller deliveries may mean greater overheads and push the price up.
If people are buying in 500l lots instead of 1000l lots then that's what they are doing. How does that affect the price? People are still buying the oil. The local supplier only loses the smaller deliveries if another supplier is cheaper. That's called competition and is just as applicable to the large deliveries.
What's the agenda behind your question? Are you trying to screw a lower price out of a supplier for a smaller delivery?
MBQ