Pasty Prices - Something to Chew Over

I am trying to get my head around this example of pasty prices that I encountered at Waterloo Station on Tuesday night. The vendor is selling a large pasty for £3.90 and “The Big One” for £4.00 - a difference of just ten pence? Why not charge more? Or at least cut the price of the large pasty to something like £3.30 to encourage people to pay the £4.00 option? Am I missing something here?

I asked the guy running the concession “what is the difference between the large pasty and the big one?” His cogent reply put me right “The big one is bigger!” I bought the large pasty because as far as I can see, there is no difference in size.