If the number of memory chips as well as the bus width stay constant, not much changes irrespective of the capacity (the changes are tiny). If you add more chips to increase capacity, yes in that case the power consumption rise. But even then, if you keep the bus width constant, it is actually much less then linear. If you put two GDDR5 chips on a single 32bit Controller port (i.e. run them in clamshell mode where each chip provides 16 bits for each transfer), they won't consume twice as much as a single chip operating in 32bit mode. Last time I checked the power consumption in clamshell mode was something like 60-70% of the maximum when operating in 32bit mode.