When you created the PDF form, was it save as "Static PDF" or "Dynamic PDF"?
In general it is best to use a dynamic XFA PDF in a digital signature solution. This becomes the best choice for workflows that require multiple signatures with a lot of data being added/changed between signatures. AcroForm and Static XFA PDF represent the document snapshot of the PDF by adding the changed data values as interactive PDF objects. While dynamic XFA PDF adds a flat snapshot of the PDF at the time of signing which is smaller in size.
It's a Dynamic PDF.
What I'm asking is why two signatures. both from a CAC cert, both to an empty document with no other changes, would be so different in size.
Neither signature has been modified with a digital image.
You mentioned that the form has been "extended" for Reader using Acrobat Pro. To eliminate this as a possible contributing factor, can you test a copy of the form with out it being extended and sign it (multiple times) using Acrobat Pro to see if the file size increase is the same?
What Target Version of Reader did you set for the form (in Designer)?
Can you post a copy of the form?