I have created a form. I use the "Save as . . .> PDF". If I add a form field and save the file the file size increases slightly. If I delete the form field, the file size decreases but it is still greater than the file size of the original file. Doing this repeatedly continues to add size to a file. It seems that the file is retaining some additional information that continues to reside in the PDF file.
Sounds crazy. For example, I have a form with a very large number of fields. The file with no form fields is around 300k. The file with a large number of form fields that I have been working on for a long period of time is about 7,000k. When I delete all the form fields, the file "shrinks" to 6,000k. The only difference between the 300k file and the 6,000k file is that the 6,000k file has been worked on and saved continuously for a period of time. They should be the same file size.
I've tried saving the file as reduced size PDF or optimized PDF. Makes no difference. In Acrobat Pro 9, every time you used the "Save as . . ." feature, it would eliminate all of the changes. Acrobat X doesn't seem to be doing it.
It can be a pain to copy the fields and get the field positions the same, so I have a script that records all of the field coordinates from the old and applies them to the new after the fields have been copied.