Lenses give an image of the object. The size of the image compared
with the object depends on the magnification of the lens.
The magnification of a lens is defined as:
In spite of what you might think magnifications don't always have
to be bigger than one. For example in a camera and in your eyes the
image is smaller than the object.
However if we look at the diagram and consider the triangles OO'P
and II'P we can see that the magnification is also equal to image
distance/object distance or v/u.
If the image is real and 7.5 cm from the lens and the object is real
and 15 cm from the lens the magnification would be 7.5/15 or +0.5,
the image is real and half the size of the object.
If the image is virtual and 24 cm from the lens and the object is
real and 8 cm from the lens the magnification is -24/8 or -3, the
image is virtual and three times the size of the object.