Three (R,R′)2Mn2O7 ferromagnetic pyrochlore systems were studied to investigate the role of the R3+ ionic size versus 4f moment on the magnetic properties of the Mn2O7 sublattice. The Curie temperature TC=18±1 K for R=Y and Lu remained nearly constant for (Y1−xLux)2Mn2O7 but the magnetization data show characteristics of spin-glass behavior in low magnetic fields, and at 5 T, the magnetization fails to reach the expected 3 μB/Mn4+ found by 0.5 T in Tl2Mn2O7 and In2Mn2O7. A frustrated, minor antiferromagnetic component apparently competes with the ferromagnetic component of the Mn4+-O-Mn4+ interactions to give a minor antiferromagnetic component to the major ferromagnetic spin alignment of the Mn4+ ions. A TC=42±1 K for R=Dy and Yb remains nearly constant in the (Dy1−yYby)2Mn2O7 system and TC increases systematically from 19 to 42 K with Dy concentration in (Dy1−zLuz)2Mn2O7, which clearly shows that a ferromagnetic interaction between the R3+-ion and Mn4+-ion spins favors ferromagnetic alignment of the Mn4+-ion spins to double the magnitude of TC by relieving the frustration of the Mn2O7 sublattice. The R3+-ion size has little effect as the Mn-O-Mn bond angle changes by no more than ∼1° with changing ionic radius of the R3+ ion.