It is known that the mechanical responses of metallic samples are insensitive to confining pressure at macroscale. As a result, von Mises elastoplasticity has been commonly used to model metals in engineering practice. With the use of discrete dislocation dynamics in this study, we explore the dislocation behavior of finite-sized copper single crystals of different sizes under uniaxial compression and hydrostatic pressure, respectively. It is found that the dislocation density approaches a stable value with the increase of hydrostatic pressure while it still keeps increasing under uniaxial compression as the size-dependent yield stress is reached. This difference is also dependent on the loading rate. The yield stress under uniaxial compression exhibits the conventional loading rate effect, while the stable value of dislocation density under hydrostatic compression increases with the increase of loading rate. Moreover, a transition from being pressure-insensitive to pressure-sensitive on the evolution of dislocation density is observed under hydrostatic compression as the sample size becomes small. These findings provide useful insights into the elastoplastic responses of metallic samples at microscale.