Why does the human eye-lens have a constant magnification for a fixed object distance?

The human eye-lens flexes to change focal length and thereby bring objects of various distance into focus. The magnification of the lens $M$ is always constant for an object with distance $d_o$ because the image distance $d_i$ is also constant as the thin lens formula for magnification implies: $$M = \frac{d_o}{d_i}$$ But this is only true if the image is in focus. If it is not, the image distance $d_i$ could change while the object distance $d_o$ remains the same and we would see the same object at a different magnification.
Q: Why does this never happen?

• Cross your eyes while observing something like your keyboard. Details of the image will appear to get smaller. – Hot Licks Mar 29 '18 at 0:30