In a galvanic cell, the reason the voltage is generated is because of the combined electric potentials of the two metals in the metal sulfate solution of the electrolyte.  If you increase the concentration of one of the electrolyte solutions, you increase the number of cations and anions (depending upon which electrolyte you increase), thus increasing the voltage potential of the cell.

In a galvanic cell, when a current flows in the circuit, equilibrium conditions are not achieved and the cell potential will usually be reduced by various mechanisms, such as the development of overpotential.  It is also important to note since chemical reactions occur when the cell is producing power, the electrolyte concentrations change and the cell voltage is reduced, the longer the cell operates.

The concentration of the salt bridge has to be such that it effectively separates the two electrolyte solutions, but porous enough to allow the steady exchange of cations and anions between the two electrolytes.