Katamari Tech Demo

...the scaling could be done with a single key frame representing the maximum scale of the ball, then send a signal to that key frame that is as strong as "current size" over "maximum size".

...as more objects are acumulated, that way you'll continue to roll smothly, wont bounce around as much, and have a more uniform shape, making it easier to collect objects...

it gets increasingly hard to pick up objects the bigger you are compared to them. I recomend, rather then using the objects themselves as the collision detection, instead have an invisible collision box thats a perfectly round sphere that gets larger...