If particles are accelerated collectively, as would happen if there is a period of collective expansion before the hadrons become free-streaming, the pt spectra of the particles would be affected in proportion to their mass ( same velocity, higher mass -> higher momentum). This is exactly what is seen in this picture.
This figure suggests that there is no collective flow in pp collisions, but there is more and more flow as systems get bigger. In this interpretation, the underlying true freezeout temperature is about 140 MeV.