There are several possible reasons the PDF output is larger than the input:
There may be a large number of rasterized fonts (Type 3 in the PDF). If you have many small PDF files, this will cause the PDFs output to be large. The statistics LOG shows you the number of fonts.
If you are mapping to the built-in Base 14 fonts, ensure that pdfsubsetfonts= is set to YES
Ensure that there are no filters set to NONE in the INI, for example pdftextfilter=none
If you are creating PDFA/1B, ensure that all fonts and color tables be inline
Graphic compression in AFPDS is not as good as PDF compression. If you have a substantial number of IOCA graphics, the output could be compressed further using PDF methods. This may not resolve the issue, as the AFP compression may be very good all ready.
If rasterformat= is set to CMYK or RGB, the color increases file sizes. Determine whether you need color in the output.
There may be a large number of inline and unnamed graphics that be being converted. Determine whether they are the same graphic being embedded many times, and whether the graphic could be reused instead of being converted and embedded multiple times.
A debug file shows the external page segments and inline graphics. You can compare this to the graphics in the output. Set the graphics to be reused if required; see the reference guide for the product you are using for more information.