Pdfcreator 3.2.2 increase file size in pdf / a format

I use pdfcreator 3.2.2 to convert the files from pdf to pdf/a format in order to digitally sign according to the Italian law I am using pdfcreator ver 3.2.2 but when I convert a pdf file to pdf / a the size increases considerably, for example: original file was 951 KB while the output file is 1.97 MB maintaining the same size of the sheet and resolution in DPI Why does this happen? Finally I would like to know if I use the Pdfcreator version plus I would get a different result?
Thanks and Regards,