The Kyuubi is like the size of the Juubi's head, so the Juubi's Bijuudama is bigger than you'd think. Also, greater size does not necessarily equal greater chakra, but greater chakra DOES tend to equal greater power. Case in point, Hashirama's Shinsusenju is larger than Sasuke's Bijuu Susano'o, yet Sasuke's Susano'o is stronger. The reason for that is the greater amount of chakra concentrated in the Susano'o. Juubi Jins, Kaguya, etc. also possess greater chakra (and their power rankings largely reflect how much chakra they have, with Kaguya as the strongest, followed by Madara, and then Obito).
There are very few instances in which superior chakra is overpowered by inferior chakra in a direct clash, and these tend to be the result of special exceptions (such as Naruto and Sasuke overpowering Obito's SO6P sword due to Obito's emotional instability and flashbacking). At any rate, avatars such as Susano'o, and attacks such as Bijuudama, generally follow the more chakra = more power rule.