There's so many variables in your question it's hard to know where to begin.
1. The size of the PM file doesn't necessarily directly relate to the size of the PDF created from it.
2. Different versions of Distiller do produce different sizes of PDF.
3. You do not need Acrobat to create PDFs from PM.
4. Your screen captures suggest the PDFs were created using PM's Export… AdobePDF…, and somewhere along the line you changed your version of Distiller. Have you installed a version of Acrobat?
5. PM is not supplied with Acrobat, which is a PDF editor and creator - only Distiller, which is the creator part. I think I'm correct in saying the latest version of PM is supplied with Distiller 6.
I'm pretty sure that PM7 came with Distiller 5
I thought the last and latest Windows version 7.0.2 came with Distiller 6. All others had Distiller 5.
The PM support info from April 2001(!) does say Ver 5.0 though: http://www.adobe.com/uk/products/pagemaker/pdfs/pm7ov.pdf