Need guidance on how to reduce size of PDF/A files

I have several colleagues who create large (hundreds of MB's) PDF's of legal documents. One of the requirements of a third party is that these PDF's be converted into PDF/A files. Here is the problem: when they perform the conversion, the PDF becomes several times larger (e.g., a PDF which may have been 100MB in size balloons to 300-400MB upon conversion to PDF/A). This impacts their productivity because the upload limit of this particular third party's site is 150MB, so they have to split the PDF/A into multiple files, introducing the opportunity for corruption. They have tried various utilities to compress the PDF/A's, but with little success. The versions of Acrobat in use are both XI Pro and DC Pro. Windows version is 10 Anniversary Edition and word processors are MS Word 2013 and Wordperfect X7.