Here's why:

* Velocity is a vector quantity, meaning it has both magnitude (size) and direction.

* Speed is the scalar (magnitude only) counterpart of velocity.

Think of it this way:

* You're driving a car at 60 miles per hour (mph) north. That's your velocity – it tells you how fast you're going and in what direction.

* The speed of your car is simply 60 mph. It doesn't tell you the direction.

So, the size of the velocity vector is the speed of the object.