Fun game that scores very well on the theme.
In level 6 I think I found a bug, I just wanted to get my ball down as quick as possible to win the level before the heavier ball even got there so I started out with a ball of minimum weight (5 in this case).
And it seems that small balls in that level do not hit the collider, but just go straight down, beating the level in speedrun times ;P
Great job, I had a lot of fun with it :)