As you clear out tiles, bears will take their place, and they'll stretch out to be the largest possible rectangular bears they can be. The bigger your bears at the end of the stage, the more bonus points you'll earn. This means you should do your best to clear large sections of the stage for your bears to sprawl out on. Of course, leaving tiles, as we've mentioned, turns them to stone, thus hampering your progress and how big your bears can possibly get.