I had to replace the fonts, but after that I could export a pdf and the CC 2019 pdf has 261,2MB, the one created from CC 2017 2MB, so not exactly the same values as yours but pretty similar.
Within the CC 2019 pdf the main issue seems to be transparency because in »Optimize PDF« it gets the size down to 3,7MB right quick …
You may want to check over on
Photoshop Family Customer Community
At least one thread exists on the issue
Photoshop CC 2018: Exporting Artboards to PDF are much larger in file size | Photoshop Family Customer Community
and Jeffrey provides a work-around in the sixth post (if I didn’t miscount).
Thank you for pointing that thread. Copying those two files from previous version actually did solve the problem!
The workaround is that you have to overwrite ArtBoards To PDF.jsx and ArtboardExport.inc located in Photoshop Presets/Scripts folder with those from older version of Ps.
So far everything seems to be ok!