Please note that this is the Narutopedia's article on Jiraiya's version. If you are looking for the article on Naruto Uzumaki's version of the technique then you should head to Sage Art: Ultra-Big Ball Rasengan.
This technique adds natural energy to the Big Ball Rasengan. The natural energy rapidly increases it to gigantic proportions, larger than the user's own body, and is created in an instant. When it impacts, there is a large shock wave unleashed that can send unsuspecting shinobi flying.
It was noted that if the Ultra-Big Ball Rasengan were to explode, it would easily carve away an entire mountain.