In Survival Mode, you have to wait a bit before there are enough smaller asteroids to really begin to feed and grow. There will be large clusters of Suns in the center so be careful of venturing too far out.
The only drawback I've found is there is no way to end the game and retain your score for mass if you "give up"
I've had a sun so large I filled up the entire screen, it just kept eating.
Well, I just found a bug. There's a point where you can no longer see any part of your sun except the middle bit, which is all yellow. And you can't end the game as a winner even though nothing can possibly eat you because you take up the ENTIRE screen and galaxy map.
 "Cosmic Crush"){kind=icon}