This study reveals that injecting a light fluid of density ρ_{b} in the recirculating bubble of a bluff body at Re≈6.4×10^{4} has a greater drag reduction potential than blowing fluid of a density greater than or equal to that of the free stream ρ. It is found that the maximum drag reduction scales as (ρ_{b}/ρ)^{-1/6}. This power law combines the ability of the recirculating bubble to diffuse the injected momentum and the effectiveness of the injection to increase the recirculating bubble length.