//found on : http://www.mographwiki.net/After_Effects_expressions_library
Apply this expression to the 'scale' parameter of your layer.
k=100; // final scale
a=5; // how quickly the bounce ends
b=20; // how many bounces it'll do (that is, the bounce speed)
x=k*(1-Math.exp(-a*time)*Math.cos(b*time));
[x,x]
