The ratio of the height of the image of an object to the height of the object is :

The ratio of the height of the image of an object to the height of the object gives the magnification(M) of the image. 

$\mathrm{M} = \frac{\mathrm{I} (\mathrm{Image} \mathrm{size})}{\mathrm{O} (\mathrm{Object} \mathrm{size})}$

It tells how large or small the given image is as compared to the object.