Users have reported enormously bloated files where the issue appears to be spurious entries in the photoshop:DocumentAncestors XMP tag. I was provided some sample files and found tens of megabytes of data disappeared from each file when I removed that tag.
The file that I was given was created in Photoshop CC 2017 for Windows.
While not absolutely certain, evidence seems to suggest that the corruption is being produced in Photoshop, under conditions that I haven't been able to reproduce. As opposed, for example to it traveling around the internet on infected files (which may well be happening as well)
See this thread in the Adobe user forums https://forums.adobe.com/message/10371400 (my comment is at the bottom as I write this.) Or this thread (look near the bottom) https://forums.adobe.com/thread/2382524?start=40&tstart=0 and this blog post http://prepression.blogspot.com.au/2017/06/metadata-bloat-photoshopdocumentancestors.html
I have the files and would be happy to pass them along to you, along with any help I may be able to provide if you'd like.
(Note that in both those forum threads, users actually discovered the problem when the bad files were incorporated into InDesign or PDF files, but the issue began in image files (the ones I looked at were Tiffs.)