You are right, it's S3, sorry for forgetting to specify it. So pity, that this fact is not mentioned in rclone docs,tough I may missed it.
But if so, another question is : I synchronized ("rclone sync") two buckets. As far as I understood, default upload chunk size is 96M. I may suppose, that small files (up to this size) will be copied not in chunks.
Is there a way to say rclone (either sync or copy) to upload files not in chunks?