The Chromium pdf printing evolves in time. The later versions add accessibility, image masking, and other implementation changes that may increase the final PDF size. We update Chromium every time when releasing jsreport, that's the reason why v2 and v4 pdf outputs differ.
There are 3 visible images in the flight ticket showcase, but every image also has an alpha mask. That's why you see 6 images in the PDF source.
You can try to enable the pdf compression feature
