Thanks for replying!
Well, I have to admit that the programming part took me sooooo long, that there's little time left for me to do the balancing. (sad)
The mass of a part is proportional to the pixels in it (with a minimum of 24), and will be reduced to 1/7 upon disjoint.
I personally love the "broken" mechanics for large planet parts, as it shows the mass of the planet very exaggeratedly. The "joined" mechanics for small planet parts satisfy me as well. However, for joined large parts and broken small parts, maybe I've pushed too far.