The emitted particles from a source produced by colliding large and small systems show a common feature known as mT scaling. Particles with larger transverse mass are emitted from smaller system sizes. In large systems such as those produced in Pb-Pb collisions, this effect has been attributed to the collective expansion of the medium. In recent years, collective-like signatures in small systems, namely p-p collision, have been observed, although the origin of such observation is still under debate. The "Hydrodynamics and the source size in small systems" aims to review the physics behind the effect of collectivity in observing mT scaling and discuss its connection to so-called collective expansion in small systems.