Pando, thanks for reply.
Yes, I understand there must be some redo.
Just see again my original post.
Populating the table with over 20,000 rows makes about 20 K redo. That's great and works as I'd expect.
But the same count of rows inserted into the same table with NOLOGGING index makes about 1,5M redo. That is the problem. It seems that subsequent maintenance of nologgingg index after direct-load insert is fully loged in redo. But (if I understand it) it shouldn't.