Before firmware v3.2 users complained that scribble output pdf is a crappy useless low resolution rasterized file. WTF only Dropbox files are vector pdfs.
Now with firmware v3.2 real vector pdfs are the new standard and users are complaining that pdf files are huge. All strokes are editable.
It is a no-win situation for Onyx.
You can try to create a "flattened copy" in Xodo app. So the pdf annotations are not editable anymore and should be merged and this should result in a smaller file size.