Converting catalog of EPUBs to PDF - becomes huge

For 'workflow' reasons, I would like to convert my catalog to PDF (for transfer to an app on my iPad). The problem is that when I convert a mid-sized catalog (< 3000 books) in EPUB format to PDF format, it results in a massive file. The EPUB catalog is 24MB whereas the PDF is 580MB.

I'm pretty sure that it's not to do with the covers in the description field -- I tried a catalog without covers and the EPUB version fell below 1MB but the PDF was still several 100 MBs in size.

Any ideas? Thanks...