The window area has a default range of -1 to +1 in both dimensions. The origin with coordinate 0 is in the middle of the window.
A standard quad has width and height of 1 with its center at (0, 0). Therefore the quad extends from -0.5 to +0.5 in both dimensions.
Resizing the window stretches the coordinate system in the same way as the window.
See Aspect Ratio on how to avoid stretching.