Classical physics is fully capable in explaining the motion of bigger objects or the objects we can see through our naked eyes on earth.
But when we know about other objects like qunata particles and celestial bodies. These are not adjusting with our theories, for studying them we introduced a new PHYSICS and that is known as MODERN PHYSICS.
That’s the peak from where our PHYSICS is divided into two parts that is classical and modern.
The answer in the easiest way is take three fruits: WATERMELON, Orange, and a grape. Now, just put these in front of you and think that for grape, the orange is bigger in size, but the same orange is smaller in size for the watermelon.
This simply indicates that the size depends on the viewer and hence it is relative.