Adding page numbers and file size

I have a simple question...

I have Acrobat XI Standard on my main computer and Acrobat 9 Standard on my laptop.

If I add page numbers to a document in Acrobat XI  I end up with a larger file  size than when I add page numbers to the same document in Acrobat 9.

This difference can be significant for large documents (1.34mb vs 1.16mb for a 140 page document, for example).

Can anyone tell me why there is such a difference between versions, and if there is a way reduce the file size in Acrobat XI?

Thanks!

Ken