Compatibility between different readers is one thing to try: maybe your new PDFs have broader compatibility than the original? This could cause ballooning file size if, for example, there's a complex effect like a gradient mesh that can be represented as one element in modern readers but needs to be broken into thousands of separate elements in older readers. I've had this problem when saving Gradient Mesh effects as EPS, never seen it for PDFs but it's possible.
If that's not it, Illustrator does sometimes bizarrely balloon file sizes. I've had PDFs that were stubbornly several megabytes in size even after deleting the entire contents of the PDF! I think Illustrator can sometimes find itself carrying dead weight that it's unaware of.
If all else fails, there are two brute force approaches to fixing bizarrely ballooned file sizes, which you could try individually or together:
- Create a new, blank document, with the same page settings, and copy and paste the contents over. Hopefully this will keep the artwork and leave behind the cruft.
- Place all the different PDFs into another application (ideally InDesign, one page per page, but you could maybe use Illustrator), then save as one PDF from there. The master application should apply its own PDF settings, hopefully cheerfully annoying all the cruft in the placed PDFs.
Assuming the problem is a load of dead weight behind the PDF, these could trick Adobe into ignoring that dead weight and just looking at the actual artwork.
Do these from the original, pre-inflation PDF, in case an earlier use of unticking the "Preserve editting capabilities" alongside generous compatibility settings or similar caused an effect to be broken into thousands of component parts irreversibly.
If even this doesn't work, chances are Illustrator is breaking something simple into something complicated the moment you first open the PDF. If you open then do nothing but save as
.ai, is that huge too?
If the text edits aren't simple enough to do in something like Acrobat, you might be able to place the PDF in illustrator (therefore not converting it into editable Illustrator format, therefore not expanding whatever effect is causing the ballooning file size), then super-impose your new text over the top.
If super-imposing isn't possible, if you've got Acrobat or similar, you could delete the incorrect text elements completely in that, save, then place into Illustrator and add new text over the top in Illustrator.