Here is a sample script that uses cubic interpolation to resize a ball. The animations will start slow and speed up to the end. You can tweak the 
p1 and 
p2 parameters of the 
vizact.sizeTo() command to adjust the midpoints of the cubic spline. Run the script and press the spacebar to toggle between growing and shrinking the ball.
	Code:
	import viz
viz.go()
ball = viz.add('ball.wrl',pos=(0,1.8,3))
growAction = vizact.sizeTo([4,4,4],time=1,p1=0.0,p2=0.1)
shrinkAction = vizact.sizeTo([1,1,1],time=1,p1=0.0,p2=0.1)
vizact.onkeydown(' ',ball.runAction,vizact.choice([growAction,shrinkAction]))