The 9 Bijuu RS are stronger. For the two RS, they were about the same size as the Chou Oodama FRS Naruto used on the Juubi here
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with the differences being RM has stronger chakra. Naruto only used 2 of these of about equal size; 1 being Senpo Chou Oodama FRS and the other Senpo Chou Oodama TBBFRS. In the case with the 9 Bijuu RS' they weren't normal size FRS which uses a normal size Rasengan in the base of it like so
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. They were called super tailed beast Rasenshuriken which made me think they were Oodama FRS but your scan says "Chou" which is ultra. Plus I don't think you took into account the power FRS gains when Naruto is adding other elements to it; think of the power differences between a basic Rasengan and FRS which only has wind chakra added. Naruto adds KKG such as Steam, Boiling, and lava to his FRS which are using Oodama (or a bit larger) size Rasengans for their base. The fact that there's 9 of them with such power puts it over the 2 RS.
Also that big explosion is with the clash of the 2 RS with Sasuke Bijuu chakra powered Indra arrow. Naruto's 9 Bijuu FRS explosion from a distance which still looks huge
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and the explosion from both Naruto's and Sasuke's attacks close up with smoke surround most of it
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