I am placing high resolution pdf's into my InDesign doc. The file size of these pdf's are about 60MB. Once I export a X1A PDF from InDesign the file size is around 4MB. My question is will these print fine, why is the file size so small.
InDesign treats placed (linked) PDFS the same as it does placed images. It references the original file upon output, then applies any in-applications transformations which have occurred, such as scaling, rotating, etc.
If your PDF is of print quality, and you haven't used any unruly transformations to the placed PDF in InDesign - such as enlarging it 200% - then yes you can use high quality PDFs and they will output as high quality. In fact, if the nature of the PDF is vector, then even enlarging it will not degrade its quality.
I often place reader-spread PDFs in a new InDesign document to create printer imposition. It's so much easier much of the time.
All of this, of course, depends upon the quality of the original PDF. You can't create a press-ready PDF from InDesign is you are placing web-resolution PDFs. The same rules or guidelines for general image quality is true for PDFs as well.