How the file size changed

Suppose I create an empty file in MS word (its size is say 21.5KB). I copy some data into this file from other file. Now the size of file increases say 150KB. Now i again empty the file and save the empty file.

Many question is why the file size is not same as of empty file 21.5 KB. It will be slightly more then that, even if all the data from the file is deleted(its empty) ?