Why do some programs use more and more memory as they run for longer periods of time?

I often use the music program Finale. When I open it up, it takes up a little over 100,000 K of memory. (it is a large program) However, as I do stuff in it, the program grows to almost 200,000 K.

Now here’s the problem: If I don’t touch the program after that and I leave it open, Finale swells up to almost 300,000 K by the next day. This causes my computer to slow to a crawl, Finale begins to stop working right, and I am forced to exit the program with Windows Task Manager. Why does Finale grow when I am not even using it, and I just have it open? Can I fix it?