If you look at the HSC electric potential table, which lists elements reduction with increasing reduction potentials-i.e. more positive as you go down the table, you see there are three elements/compounds between magnesium and zinc, while there are about 5 element/compound between copper and silver-essentially more reactive metals when used in a galvanic cell does not produce greater voltage, but rather the further spaced the metals are on the reduction potential-the greater the difference of their reduction potential-which will increase their tendency to donate or accept ions respectively, the higher the voltage of a galvanic cell-which essentially refers to the transfer of electrons in the external electric circuit, to illustrate this point further, say two very reactive metals, sodium-placed in water, will burst it into fire, and magnesium-readily burns with spark into a white powder-the first has a reduction potential value of -2.71V, but the second has reduction of -2.36V, so magnesium will be reduced and sodium oxidised, giving only 0.35V-which is a lot lower than your less reactive(the two metals comparative) combinations above, it is the difference in reactivity that matters, not the reactivity of the metals itself
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