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

A PDF file usually stores an image as a separate object (an XObject) which contains the raw binary data for the image.

This is not generally an image in the sense of a TIFF or a JPEG – it is the binary data for the pixels, the colorspace used for the image and additional information.

The actual pixel data can be compressed and one of the compression formats (DCTDecode) is the same used as in a JPEG. If you save this data, it can be opened as a JPEG file, but it may need altering to include the colorspace data.

Depending on the tool used to create the PDF your HEIC file is “converted” and included in the PDF generated.

And depending on the parameters used when creating said PDF no compression might be used - besides the obvious, that is that HEIC is much more efficient for file size than the compression algorithms currently supported by Adobe’s PDF (assuming you want to keep the pixel dimensions and dpi of you original images when included). So you might end up with a large file size for your PDF.

PDF is not a good image storage format. Why not use JPEG, TIFF or PNG?

EDIT: if you have a folder of HEIC files on your Mac, launch the terminal (Zsh) and create JPEG copies of your HEIC using sips:

for i in *.heic(:r) ; sips -s format jpeg "$i.heic" --out "$i.jpg"