Fun game, managed to get to 6k lbs over the course of 2.5 hours.
One bug that I did notice with the combining logic: When there's two pairs of collisions between combinable balls on the same frame, *two* balls of the next size up are created (which then usually instantly form into the next size up).
See this Codebullet video for how this could go bad: www.youtube.com/watch?v=QbwgQSwMSGM @ 4:00. In that clip, two collision pairs create two fruit, which immediately combines with each other and a different fruit (three collision pairs), which creates exponential growth of collisions.
You could try to detect when there's a single fruit colliding with multiple others and only spawn one fruit from all collisions combined.