The 3.1m^2 loading space is based on: 1.72x1.44x1.25. These are the maximum dimensions of length, height and width. In practice, this is where the rear wheels start.
The floor area is 170x115x114 (dxwxh) cm. This is the width between the rear wheels, from closed door to passenger seat. The backrest of the passenger seat run obliquely to the back, after about 15 cm height. As a result, the length of 170 cm decreases as you pile up further in height.