Mac OS Creating PDF's weirdly large in size since I changed my iPhone
The Create PDF Quick Action (or similar Automator actions) don't have any 'controls' for the way the PDF is created, (of the kind you find in Acrobat or the PDF Export dialog of some apps). It just 'wraps' the image data in a PDF format. (I think it may also stick the image on your default printer page size.)
It's possible that the HEIC format, being newer, contains compression that the PDF format can't use, so it has to use a less efficient method. How large (pixels) are the images?
It's certainly possible to resample the image data, or increase the compression used, of a PDF: you could create a new Quick Action to create the PDF and then downsample or compress it.
What's the purpose of converting them to PDF? Are you collating images together into one file?
There are other (dare I say better) Quick Actions for PDF manipulation, including one to Create a PDF from images, which is faster than Apple's own and preserves the images' dimensions.
https://github.com/benwiggy/PDFsuite