Size selection was demonstrated to suppress Ostwald ripening of supported catalytic nanoparticles. When the clusters are subnanometer in size and highly fluxional, such as Pt on rutile TiO2(110) surface, this paradigm breaks down, established theory sintering needs a revision. At temperatures characteristic catalysis (i.e. 700 K), sub-nano thermally populate many low-energy metastable isomers. ...