I've only ever seen that happen when re-exporting using Quartz filters, where the whole point is to re-encode, and saving an e.g. 300-page PDF will take several minutes, with a progress bar.
If you're just using Preview to rearrange/combine/etc. pages, no re-encoding happens and file size stays the same.
Believe me, I do a lot of PDF management in Preview. Even to the point of creating my own Quartz filters in the ColorSync Utility precisely so that I can intentionally re-encode images when I want to. (Because the Apple-provided Quartz filters are hard-coded to abusrdly high or absurdly low resoltions.)
It’s been years since I stopped using Preview for PDF files, generally using Skim for reading and pdfpen for editing them. Perhaps something changed since then? That is good news, if so.
I should have tried it before replying, but did not have easy access to my laptop. Now I redid the experiment. Starting with a 24 page scanned pdf, I deleted the first page using Preview, then saved and closed the file. It swelled from 2 MB to 6.7 MB. Looking into the file, I see that all the images in the original are encoded with CCITTFaxDecode, while the images in the edited file are encoded using FlateDecode. I cannot see anything relevant to the issue in the preferences.
If you're just using Preview to rearrange/combine/etc. pages, no re-encoding happens and file size stays the same.
Believe me, I do a lot of PDF management in Preview. Even to the point of creating my own Quartz filters in the ColorSync Utility precisely so that I can intentionally re-encode images when I want to. (Because the Apple-provided Quartz filters are hard-coded to abusrdly high or absurdly low resoltions.)