Safari download file size problem

Lately I have been downloading a lot of files via safari.Whenever I download them, the correct file size (the one below the file name in the downloads window) comes out after downloading.However, sometimes when I leave it there for a while, or when I quit safari or turn my macbook off, the file size suddenly changes to a lower size in the downloads window.Are there any effects of this?Also, how do I avoid this.

I am currently using safari version 4.0.4

Well, is it causing a problem? I mean, can you still use/open whatever it was you downloaded? If so, don't fret about it. Maybe it's one of those things that just doesn't need an explanation.

Yes, I can still open the files ,but i'm not really sure what caused this problem.Also,I'm not really sure if there are any effects,that's why i'm asking,but i really do hope it's one of those things that don't need an explanation.

Problem? What problem? You can open the thing you downloaded. And that's a problem? Ignore the size change.