Why are there fabric sizes and finished sizes for futons?

In Japan, the size of a futon is customarily indicated by the size of the fabric before the cotton is filled.
It is the size of a flat, rectangle piece of fabric.

When that fabric is stuffed with cotton, the size seen from above becomes smaller as it expands and becomes taller. This is the finished size. That's why we describe both sizes.

As the futon is used, the cotton is compressed and flattens out, so it looks a little larger from the top, closer to the size of the fabric.