Increasing concentration of electrolyte increases the number of ions in a particular volume.
As a result, electrical conductivity of the solution also increases.
You may call it decrease in resistance too, because resistance is just the inverse of conductance.
Thus, for a given voltage, increasing electrolyte concentration increases the current density upto a certain optimum level.
Beyond that optimum concentration, however, overpotential and viscous drag of ions offsets the effect of increase in number of ions. That is why, in most of the cases, a saturation level is reached and electrolplating is carried out at a concentration below that level.