This is because of angle of vision. When you see the object at a longer distance, it appears to be small and if the object is near, it appears to be large. Actually when the object is far away from the focal length that is 25cm of your eye, it appears to be at the longer distance, if the object is exactly at its focus it appears to be of the same size and if the object is more closer to our eye that is less than its focal length then it appears to be small. And also it because the minimum angle of the eye to view the object clearly and distinctly is 60*(degrees). If the object is near the eye you just view the object as big as you think. But when the object is far from the eye and at more angle you view the object as small.