I have several questions:
1. What is the size of the original (before the first signature) file?
2. Do you sign with Acrobat X and Acrobat XI at the same time (you may get different CRLs with different lengths at different times)?
3. What is the PDF size after the first signature using Acrobat X?
4. What is the PDF size after the first signature using Acrobat XI?
5 and 6: the same as 3 and 4 but after the second signature?
It does not make sense to me that with Acrobat X you get 2 signatures with LTV and the total PDF size is just 2MB and with Acrobat XI you get 2 signatures without LTV and the total PDF size is 4MB. This doesn't make sense. Are you sure you're signing the same PDFs with the same credentials at about the same time in both cases? Or did you mean that PDF size increases by 2MB in the Acrobat X case and by 4MB/10MB in Acrobat XI case? I'd prefer to have the total PDF sizes not the increases.
Another test you can do is this. Turn off "include revocation information at time of signing" on Acrobat preferences, sign your sample PDF with the same 2 signatures, then right-click on each signature and select "Add Verification Information" command for each signature (this is a new way to add LTV available in Acrobat/Reader 9.1 and later). Then save this PDF and check (and report back to this forum) the PDF size. You cannot use this LTV method if you expect that some users will open this PDF with Acrobat/Reader versions prior to 9.1. Those versions will not use this LTV type.
Sorry for the late reply.
Also, I did not pursue this further as the problem hasn't yet occurred with other files, nor with other id-cards.
If it does, I'l come back and run these tests.