This work reports the synthesis and structures of six new AB(HCO₂)₃ (A = Rb⁺ or Cs⁺ and B = Mn²⁺, Co²⁺ or Ni²⁺) frameworks containing the largest monoatomic cations on the A-site. RbMn(HCO₂)₃ is found to adopt a distorted perovskite framework with a 4¹²·6³ topology and a mixture of syn–anti and anti–anti ligands, while the remaining compounds adopt a chiral hexagonal structure with a 4⁹·6⁶ topology. The structures of these frameworks clarify the effect of ionic size on the formation of the five known architectures adopted by the AB(HCO₂)₃ frameworks, which have attracted attention as a new class of potential multiferroics, and in particular the chiral hexagonal structure within this. This also highlights the role of molecular A-site cations in stabilising the 4⁹·6⁶ topology for frameworks where such cations are too large or small to support this structure on the basis of size alone, possibly due to hydrogen bonding. The magnetic properties of the RbB(HCO₂)₃ and CsMn(HCO₂)₃ frameworks are also reported with the Rb⁺ compounds featuring weak ferromagnetic behaviour and the latter being purely antiferromagnetic. In conjunction with a comparison of the other isostructural AB(HCO₂)₃ frameworks we find that compounds adopting the 4⁹·6⁶ topology have much higher magnetic ordering temperatures than those with the RbMn(HCO₂)₃ structure, highlighting the importance of understanding the structure–property relationships of the ternary formates.