Good job, I didnt notice that. Tobi probably had the Bijuu extracted and resealed into their respective Jinchuuriki back then already. Or he simply didnt want to waste Bijuu-Chakra.
If the current theory about Gedo Mazo being the Ten-Tails' body is correct (And I think it is) then Gedo Mazo itself doesnt have any chakra and therefore needs another chakra source to do anything (Nagato vs Hanzo)
Tobi might have provided his own chakra back then.
When Tobi summoned Gedo Mazo to take Kinkaku and Jinkaku, the Gedo's eyes were all closed.
As soon as Nagato summons Gedo vs. Hanzo, Gedo Mazo drained Nagato's life force or chakra making him look older, skinnier, unhealthier. In other words, Gedo needs some type of synchronization or host if not Tailed-Beasts' chakra.
Nagato being an Uzumaki may have been a perfect pawn for this synchronization seeing as how Uzumaki are famous for their chakra and long lives.
I personally don't think Gedo Mazo is more powerful without the Tailed-Beasts' chakra.