0.003 is an arbitrary number..
I chose to only scale within a certain area and just looked at the size of the sprite when it touched the top of that area and what size I'd like it to have, so the visual appearence made me decide that number I guess (It's been a while since I made this..)
I guess you could make the scaling based on an expression, taking into account the max y-value, min y-value, max scale and min scale, but this example was more of a proof-of-concept than an actual ready-for-publishing kinda thing..