I believe this is a new issue with a recent update. I've never had this issue before.

The attached pdf file should be renamed to an ai file. For some reason I am not allowed to attach an ai file. That doesn't make sense to me as this is a bug report platform for ai.

If I create a document that has a linked image used in an opacity mask, illustrator is not compressing the image when I save it. I'm seeing a file end up being, for example, around 8MB when it should be around 600KB. I am downsampling and using JPG compression for both color and greyscale bitmaps.

If I release the mask, and save the [otherwise] exact same file, the file size is greatly reduced.
with mask: 8MB
without mask: 600KB

Some observations with experimentation:

If I save as PDF with mask:
"Preserve Illustrator Editing Capabilities" checked: 8MB

"Preserve Illustrator Editing Capabilities" unchecked: 600KB

If I save as AI with mask:

"Create PDF Compatible File" checked: 8MB

"Create PDF Compatible File" unchecked: 600KB

I believe this is a new issue as I have been creating files this way for several years without them being bloated like this.

Windows 7. Illustrator CC (newest).

The Attached file:
Saves as ai to 630KB. Saves as pdf at about 14MB. Simply releasing the opacity mask then saves the pdf to 836KB.