The rectum is more spacious than the anus.
It is surrounded by muscles. But these are not as powerful as the internal- and external sphincter surrounding the anus.
The rectum is tube-like. But it’s curved. The first part tilts toward the belly. A few inches in it changes direction and turns toward the back, although this curve is less pronounced. After a few inches it changes direction again and turns toward the belly.
You should be aware of the shape of the rectum when you insert any object into your anus or into the anus of someone else. Changing the angle of entry might makes the difference between pain and pleasure.