Isn't the rear space of this car small? Why do people say its rear is narrow when the body looks so long?

The Velar's space is taken up by the rear end. A longer car doesn't necessarily mean more space; for the back seat, you should look at the wheelbase!

The trunk is spacious, but it takes up the rear seat space. This is the most critical shortcoming of the Velar [I want to be alone]. The central console has many minor issues.