I get the strange feeling that even if I uncheck the previously used microexporter, a render also adjusts the object. Hmmm. I had made an obj of just the sea, imported and attached a new water shader without displacement, also enabled my sea bottom fake stones again, rendered fine. But when I looked at the obj it had changed in size and comprised of just the latest crop, and also had the fake stones of seabottom.
I did use a normal render, not render to disk.
Btw, I couldn't load a 2.4 gig obj, so I decreased mpd to 0.25, kept size of total render at 1800px wide. Then the first obj was 250MB (before it got mysteriously adjusted).