Hi
I think the thing to note here is that as you use VP the cache becomes filled with essentially copies of all the clips used and index data of relating to them. (Around 5 files per clip and occasionally more.) Even if you have finished a particular project and started another, the store of data created is not removed so large numbers of files can be present that are mostly redundant. This can cause problems with the functioning of VP as it has much more data to sort through.
If you have saved your project regularly as a ,vpj file and particularly when all the editing is completed and exported, those cache files are no longer required and can be deleted. The small.vpj file you create when you save your project is all that is required to restore the project from the original raw files provided they are still on the computer or can be reloaded back. It is a longer process however as the cache has to be rebuilt but If the cache is not regularly cleared of unused files it could contain hundreds + of files and use up a lot of space.
Nat