[en] Highlights: • We investigate the relationship between ion size and mobility. • The size of the molecular ions are not point charges. • The mobility varied with the molecular size. • For linear molecules, the mobilities are always lower than the polarization limit. Mobilities of polyatomic molecule ions in helium gas were measured at 85 K and room temperature using a low-temperature drift tube. They differed from the typical behavior shown by atomic or small molecular ions in helium owing to the influence of attractive interactions among the molecular ions and also their size and shape. To investigate the relationship between ion size and mobility, we measured the mobilities of CH₃O⁺, C₂H₅O⁺, and C₃H₇O⁺.