Mac OS Creating PDF's weirdly large in size since I changed my iPhone

I have recently bought an iPhone SE and switched to HEIC format for the pictures. I am sending the files to my Mac (Early 2015 Pro, Mojave) with AirDrop. Even though the sizes of images are just shy of 3 megabytes, the "Create PDF" function under "Quick Actions" is creating ~15MB per page PDFs which is kinda painful in quick upload situations. How can I resolve this enlargement issue of Preview creating PDFs? Thank you beforehand! Cheers, ~bio