Let's suppose that a pub and a filling station are side by side. Further, let's assume that they're making transactions at the same rate. i.e. every time someone buys a pint in the pub, someone pays for fuel in the filling station. Lastly, let's assume that they've got the same overheads in terms of rent, rates, staff wages etc.
The publican calculates (for the sake of this example) that he has to make �1 profit from every transaction. The owner of the filling station concurs in this view.
However, the filling station averages 20 litres of petrol in every sale, while the pub is selling one pint of beer each time. That means that the publican has to add �1 to the cost of every single pint, while the filling station only has to add 5p to the cost of a litre of petrol.
So, when liquids are sold in large quantities (as with petrol), it's possible to make a reasonable profit with only a small mark-up. When only small quantities are sold in each transaction, a far bigger profit margin is required, resulting in a much higher price per pint/litre.
Chris
PS: There are, of course, loads of other factors which would need to be considered for a detailed analysis of your question (including different rates of duty) but the essence of my answer holds true.