We have big enterprise projects they normally involve copying data from a source database to a destination database and then setting up a number of additional applications that sync this data etc.

The last project contained 250,000 items (rows of data). The next project will only contain 4,000 items. Project managers / business people believe the project should be 1/10 the time to complete because its only a fraction of the size of the last project.

What is a good analogy I can use to explain that writing code to transfer data from one system to another takes the same amount regardless of the number items - writing it for 1 item or for 100,000,000 will take roughly the same amount of time from a programming point of view.

Tell them it's like building a new four lane highway to a remote part of the country. Whether that road gets used by 100 cars a day or 1000 cars a day, the effort to create the road will be about the same. 

Granted, if it's going to support 1,000,000 cars a day you'll have to make the road a little more robust, but regardless, you're going to have to cut down the same trees, blast through the same mountains, level the same amount of dirt, and these activities are pretty much a fixed cost no matter how many cars use the road.  
