Taking a page from Katamari Damacy's book, maybe new obstacles could be revealed as you grow larger. Like you'd need to avoid hitting birds or people riding a ski lift, etc. Or else you could just have the playable area expand and reveal new paths at a different scale as you grow larger. Though features like that would definitely not be trivial to implement because you'd need some way to dynamically switch level elements. Or maybe, if you get hit when you are big enough, the snowball could split into several smaller ones that you could control simultaneously (like the Double Cherry powerup from Super Mario 3D world), though only one ultimately needs to make it to the end. They could even recombine together to give players a chance to regain their lost size. The game is already polished and refined mechanically, but these might just be other things to explore.
Or you could go for something completely nonsensical... Like if the snowball gets big enough, smaller snowballs start orbiting it, or it accretes enough mass to become a small planetoid and it starts generating a magnetic field by rolling down the mountain... I'm mostly kidding about those, but since the game is already kind of goofy, maybe you can explore the behavior of the growing snowball in a more non-literal or wacky way.
In terms of presentation, it would also be cool if the music changed dynamically with speed or size. Even without changing anything else, I think that could really add to the tension of rolling at high speeds!