Why are the magnification properties of microscopes and telescopes defined in term of a ratio of angles and not in terms of the ratio of sizes of objects and images?

Microscopes are used to see small organisms such as cells, and some tiny microorganisms such as

fungi, algae etc.

Telescopes are used to view distant objects such as stars, moon and observe other planetary

motions.

Hence, the main variation is the distance of the objects, which varies in these instruments.

Also, due to some physical factors, there is a relative change in heights not in the angles which the

light emerging from the objects subtends on the lens.