Limiting the amount of ram goes a long way in reducing power consumption. It takes constant voltage to keep the cells alive, even when the device is locked and seemingly doing nothing.
The iPad really does have great battery life on the other hand.
I may be way off base but I think on iPhones/iPads RAM isn't a concern so much as processes sitting in the background keeping the CPU and radios doing cartwheels indefinitely for little to no reason, as they tend to on desktop operating systems. By throttling or quitting backgrounded apps they've made it much more reasonable to deliver promised numbers on battery life.
The iPad really does have great battery life on the other hand.