I've noticed that if I save a document that used images as a PDF in Illustrator, and then open the PDF in Photoshop (300dpi CMYK) and then save again as the same type of PDF, I get a file that's about half the size of the Illustrator PDF.

PDFs I create in Illustrator using only vector graphics are much much smaller, and when I open those up in Photoshop and save again, they are much larger than the original Illustrator PDFs.

To be clear, I am disabling the ability to further edit in Illustrator when I save.

Can anyone tell me why this is? I stumbled upon this by accident from trying a few things out of necessity to meet a deadline.