Someone posted a picture a while back of woman putting her Kindle on her bookshelf which was filled with Kindles.
Of course ebooks take up space. A Kindle with 3000 ebooks on board is almost half the size of a smallish paperback. Memory cards, massive things that they are, are 2 or 3 times thicker than a postage stamp. I keep my books on a portable hard drive which is about 3" by 2" and nearly 1/2" thick. That's a huge amount of space for only 78,000 books. Of course my backup is on a thumb drive the size of a thumbnail.
Barry