By chance I happened to make PDF/A-1b files from both doc and docx Word files with the same complex source material (using the Acrobat 8.2.3 "printer"). The docx PDF took much longer to produce and ended up much larger in size (about 80% bigger). Repeating the comparison with other complex Word files produced comparable discrepancies. I was "printing" from Word 2010 and lack a platform to check to see if Word 2007 does the same.
One wonders what meaningless (hopefully) docx information occupies the extra space, and also perhaps about the reliability of the PDF/A-1b as an archival format. It would be good to know if Acrobat 9 resolves the issue.
PDF/A files are always larger than their PDF counterparts as the standard requires complete font embedding and limits the compression we can apply to binary streams - but directly comparing DOC and DOCX inputs will never be simple, as Word handles even visually-similar content in very different ways when outputting to print.