  Mechanical deformation of nanopillars displays features that are distinctly
different from the bulk behavior of single crystals: Yield strength increases
with decreasing size and plastic deformation comes together with strain bursts
or/and stress drops (depending on loading conditions) with a very strong
sensitivity of the stochasticity character on material preparation and
conditions. The character of the phenomenon is standing as a paradox: While
these bursts resemble the universal, widely independent of material conditions,
noise heard in bulk crystals using acoustic emission (AE) techniques, they
strongly emerge primarily with decreasing size and increasing strength in
nanopillars. In this paper, we present a realistic but minimal discrete
dislocation plasticity model for the elasto-plastic deformation of nanopillars
that is consistent with the main experimental observations of nano pillar
compression experiments and provides a clear insight to this paradox. With
increasing sample size, the model naturally transitions between the typical
progressive behavior of nanopillars to a behavior that resembles evidence for
bulk mesoscale plasticity. The combination of consistent strengthening, large
flow stress fluctuations and critical avalanches is only observed in the {\it
depinning regime} where obstacles are much stronger than dislocation sources;
in contrast, when dislocation source strength becomes comparable to obstacle
barriers, then yield strength size effects are absent but plasticity avalanche
dynamics is strongly universal, across sample width and aspect-ratio scales.
Finally, we elucidate the mechanism that leads to the connection between
depinning and size effects in our model dislocation dynamics. In this way, our
model builds a way towards unifying statistical aspects of mechanical
deformation across scales.
