  With decreasing system sizes, the mechanical properties and dominant
deformation mechanisms of metals change. For larger scales, bulk behavior is
observed that is characterized by a preservation and significant increase of
dislocation content during deformation whereas at the submicron scale very
localized dislocation activity as well as dislocation starvation is observed.
In the transition regime it is not clear how the dislocation content is built
up. This dislocation storage regime and its underlying physical mechanisms are
still an open field of research. In this paper, the microstructure evolution of
single crystalline copper micropillars with a $\langle1\,1\,0\rangle$ crystal
orientation and varying sizes between $1$ to $10\,\mu\mathrm{m}$ is analysed
under compression loading. Experimental in situ HR-EBSD measurements as well as
3d continuum dislocation dynamics simulations are presented. The experimental
results provide insights into the material deformation and evolution of
dislocation structures during continuous loading. This is complemented by the
simulation of the dislocation density evolution considering dislocation
dynamics, interactions, and reactions of the individual slip systems providing
direct access to these quantities. Results are presented that show, how the
plastic deformation of the material takes place and how the different slip
systems are involved. A central finding is, that an increasing amount of GND
density is stored in the system during loading that is located dominantly on
the slip systems that are not mainly responsible for the production of plastic
slip. This might be a characteristic feature of the considered size regime that
has direct impact on further dislocation network formation and the
corresponding contribution to plastic hardening.
