I noticed that with each rewrite of a PDF the size increases up to a final size.

But let me explain what I did:
1) I have two different starting files to test with.
File P is created with PDFCreator (109 kB in size), File A with Adobe Distiller (that one is 29 KB).

2) I dragged each file seperately onto the PDFCreator window and saved it as PDF.
Result: File P goes up to 841 KB, File A to 56 KB

3) I dragged the files that were created in step 2 onto the PDFCreator window and saved them as PDF.
Result: File P stays at 841 KB, File A goes up to 328 KB

4) I dragged the files that were created in step 3 onto the PDFCreator window and saved them as PDF.
Result: File P stays at 841 KB, File A stays at 328 KB

That increases would only be understandable with the Adobe Distiller file A which might have had other settings than the PDFCreator when it was initially created. But that would only be the first rewrite; why does is increase again?
And what about File P? Same program, same settings and seven times the size than the first file?

Does anybody have an idea what's going on here?