Consider an object at infinity. The rays coming from it are parallel to each other. Let one of the parallel rays pass through the focus $F_1$ of a thin lens, and let a second ray pass through the optical center $O$. The ray which passes through $F_1$ becomes parallel to the principal axis after refraction and the ray which passes through the optical center does not suffer any deviation.
We therefore get the image as formed at $F_2$, inverted, real and highly diminished.
My question is: without comparing the size of the image with the size of the object placed at the infinity, how can we say that the image is highly diminished?