Creating a pivot table is very different from simply opening an excel file. Conceptually (it may be more optimized internally) it tries to create a cube of all

Creating a pivot table is very different from simply opening an excel file. Conceptually (it may be more optimized internally) it tries to create a cube of all possible results with all others. (An OLAP cube.) So if you have columns 1, 2, 3, 4 and rows 1, 2, 3 it will create a cube for a combination of all dimensions.. So for all rows you will have all columns instantly queryable but also 1-2, 1-3, 1-4, 2-3, 2-4, 3-4, 1-2-3, 1-2-4, 1-3-4, 2-3,4, 1-2-3-4 so that you can query any combination of rows/cells and instantly get your result. As you can imagine this cube gets exponentially bigger as you add dimensions (columns) and will quickly kill excel if you load in too much stuff.