Try this: Edit > Preferences > Security > Advanced Preferences > Creation > Include signature's revocation status when signing (deselect)
Yes, that's it.
So now you've isolated the cause with George's help you have a new conundrum to think about. The question at hand is do you need the signature to be validated in perpetuity? If you do you need all of the revocation information embedded in the PDF file. Otherwise, Acrobat (and when I say Acrobat I mean both Acrobat & Reader) will have to download the revocation information every time the signature is validated. That may not seem like a problem unto itself, but eventually as certificates expire Acrobat cannot be sure that the revocation information covers the certificates under test and the the signature validation will fail.
What you have to ask yourself is the long term signature validation more or less important than the increase of file size?
Finally, Acrobat 9 had a 2Mb limit on file size growth which in this case would prevent the revocation information from being embedded, With Acrobat 10 we deemed the long term validation of the signature was more important then limiting the file size growth to 2Mb (this was based on hard drive space moving into the terabytes) so the restriction was removed.