I created a pdf with Prince, but what are the reasons for the large file sizes?
How can I fix?

pdf file size : 22,263KB

Most likely fonts or images. Does the PDF contain any large bitmaps, or does it use a CJK font?

4mb size when I use another converter.
There is a picture inside the PDF.

How big is the original image file, and what format is it?

images ~3.4 mb format:png or jpg

images ~3.4 mb format:png or jpg

images ~3.4 mb format:png or jpg

JPEG images should be embedded in the PDF exactly as-is, unless there are some SVG or CSS filters being applied. PNGs may increase slightly, but it is surprising for it to blow up that much, would you be able to email me (mikeday@yeslogic.com) the image file for testing?