Automator creates PDF of unusually large size (12.5 MB vs 84.6 MB)

The bad news is that if you want minimal PDF size from multiple images then it will have to be the interactive Finder approach, with n-tuple image selections followed by Create PDF. There is no fix for whatever that Automator action is doing internally to maximize PDF size.

The Finder's internal plug-in, Create PDF, and Automator's New PDF from Images action likely use two entirely different approaches to producing a PDF from images, based on your results.

One approach to PDF generation uses the comparatively simplistic PDFKit framework code to initialize each new PDF page with an image, append those pages, and then write the resultant PDF to disk. The other, and more powerful, yet harder to code Quartz 2D graphics context approach is likely what the FInder is using with its Create PDF plug-in.

Then send feedback directly to the Automator product team, as no one in the communities can fix the issue causing this discrepancy. Use a link to your post here. The product teams neither read nor participate in these public communities.

Using Automator, I generated a large (286MB) PDF from a collection of images (jpg, png, camera RAW). I then wrote an AppleScript/Objective-C application that separately applied three custom Quartz filters designed to reduce a PDF while attempting to retain quality. In each case, the best reduction that I obtained was down to 270 MB.