Let's see the spread of the graph of the quadratic function "y=ax^2".
Observe the graph of "y=ax^2" when a=1 , when a=2 , when a=3 , when a=4 ,and when a=5.
Next observe the graph of "y=ax^2" when a=-1 , when a=-2 , when a=-3 , when a=-4 ,and when a=-5.
Let's see that when a>0 , the spread of the graph of "y=ax^2" becomes narrower as the value of "a" becomes larger.
And let's see that when a<0 , the spread of the graph of "y=ax^2" becomes narrower as the value of "a" becomes smaller.