An AI 29.5 phenomena that continues to persist and be unresolved - Ballooning file sizes!

So this issue continues, for some unknown unsolved mystery ..

Say I have an AI file, it's got some CMYK photos that are embedded. I save it as a 'Press-ready' PDF, with 'Preserve Illustrator editing capabilities' checked. It saves out to a 11 meg PDF. Yay! Prefect size for emailing.

But wait.. oops.. I forgot i needed to edit a graphic in the AI file. So I go back to same AI file, change say a simple opacity on a graphic, do the EXACT SAME save as a Press-ready PDF, same settings as before, and now the PDF balloons to 297 megs!!!

Pardon my French, but wtf. Makes no sense. It's as if Illustrator thinks about it and says to me - sorry for my oversight on the first PDF save, it's actualy quadruple the size.. my bad.

Any and all insights appreciated.

And btw - I cannot share this file or screenshots w tech support here - it's proprietary. 🙂