I think we may have gotten sidetracked by whether the images were embedded. The issue I was trying to identify was the change in the PDF structure between Illustrator versions.

Using the same document with an embedded bitmap, Illustrator 29.8 exported the PDF efficiently: all duplicate placements referenced the same image XObject, so the bitmap data was stored once and reused at different coordinates. In Illustrator 30.3, the PDF contained a separate image XObject for each duplicate instance, even though the XMP asset metadata still identified them as originating from the same embedded source image. That’s what caused the dramatic increase in file size.

I wasn’t aware Illustrator had already been updated to 30.6 until you mentioned that was the version you were testing. After updating and exporting the same document again, I found that 30.6 now behaves like 29.8: the duplicate placements reference a single image XObject instead of embedding separate copies.

I couldn’t find any mention of this correction in the release notes for 30.4, 30.5, or 30.6, but it appears the problem was corrected somewhere between 30.3 and 30.6. It’s now working properly for me in 30.6.

Thank you for taking the time to test it.

Interesting.
So I grabbed your .ai file, opened it, and discovered an image with 2 Transform effects applied to form a 3×3 grid... the image is embedded, not placed — you never said these were embedded in the original report.
Then I exported one PDF like this (5.4 MB), and then relinked the embedded image to the 'noise800px.png' and exported another one (1.76 MB).
I tried to Expand Appearance before exporting PDFs, just in case (mostly same results).

I tested this again with Ai 2024: 5.26 MB with embedded duplicates, 3.41 with placed ones.
What are your results?

This happens with any bitmap image regardless of the file format.

I used the Default PDF Preset with "Standard:None", and Compatibility: Acrobat 7(PDF1,6)" in both versions of Illustrator. I have tried all the different Standards (PDF/X-***) and the file size only changes slightly due to the Compression settings changing.

The Compression is Zip and set to not downsample.

XMP asset manifest in both PDFs identifies only one source bitmap, using the same Photoshop document ID and instance ID. So Illustrator 2026 still knows that all nine placements originated from the same embedded image. It simply doesn't reuse that image when constructing the PDF.

I tired to upload the files here but either something is wrong with the uploader or the file size that can be uploaded is tiny.

I put some smaller test files that I have been using in here:
drive.google.com/drive/folders/1Q4O-4C7ViGEkQ_qEVXOs_KSlg84IYJTO?usp=sharing

Not seeing this when I try to replicate it.
I grabbed a PNG with noise, 3000×3000, 8.58 MB.
I tested this in the latest GA 30.6, Beta 30.8, older versions 21 and 28.
In all 4 I made 30 copies, and saved using the [Illustrator Default] preset.
I got similar file sizes of 9 MB (ranging 9227-9249 KB).
If I use PDF/X-4 instead, I get 0.8-1.16 range instead.
Please tell more:
— Which preset / settings were you using, meaning 'comparable settings'?
— Does it happen with any PNG placed?
— If only the specific PND does it — can you share it?