I've come across a weird behavior regarding the output file size when exporting/saving files from Illustrator CC.
I have an image with several bitmaps (i.e. raster images/photographs) where I added some annotations, markings, paths, etc. The photograph occupies the whole artboard (which is set to my desired output size), vector features are therefor only on top of the photographs.
Now when I export this as JPEG the file turns out to be around 10 MB large. However, if I save the image as PDF and select JPEG compression, the image only turns out to be 3.5 MB large.
How can that be? Quality should be the same, detail is set to 300 dpi in both cases. I thought if the PDF embeds the JPEG, it should be more or less the same size, and adding PDF container overhead, the PDF should rather be bigger than the JPEG. How can one explain the ~60% smaller file size?