Another thing I didn’t see mentioned is the positioning of the strings and the opening of the piano lid. In a grand piano with the lid open, the strings are slightly above the base of the piano and the sound bouces up to the lid and out to the audience. In an upright piano with the lid open, the sound bounces between the walls of the piano before bouncing off the lid toward the audience. This will lead to a difference in tone and make the upright sound a little more stuffy than a grand piano.