What are you using for the signature's appearance?
I'm using the default, ARX Signature. Is it more efficient to use plain text or an imported graphic?
It's hard to say without knowing ore about the ARX signature, but I've seen people use large (in terms of bytes) bitmaps for their signature appearances. Try plain text if you can and see if it makes a significant difference.
I tried using a plain text signature and it made essentially no difference. Original file size after reduction 25.7k; size after plain text signature 1.82M.
I have submitted the question to ARX but have not yet received a response from them.
Create a self-signed digital ID in Acrobat 9.x and sign the doc. If you notice that the file size is not increased as much as it is in your workflow above, then the problem is not related to Acrobat but rather CoSign.
This worked for me to eliminate the large file size when adding a digital signature to a document ...
Acrobat digital signature adds 700KB to .PDF file size
answer (enhanced a bit) ...
2. Re: ACROBAT DIGITAL SIGNATURE ADDS 700KB TO .PDF FILE SIZE
Mar 31, 2009 2:58 AM
go to Edit > Preferences > Security
click on Advanced Preferences, then select the 'Creation' tab.
De-select 'Include signature's revocation status when signing'
Explanation: the extra data is the certificate revocation list (CRL) list that gets embedded with the signature if the above preference has been selected.
This also removes long-term validation capability. Signatures without rev info embedded in the PDF cannot be validated after the expiration of the signer's certificate.