More pixels means more circuitry in front of the backlight blocking light.
But to what degree? Blocking 2% vs. 1% of the light probably isn't noticeable; blocking 50% vs. 25% probably is. This might also be something that improves over time as process improvements reduce the overhead.
Potentially a lot. The iPad 3 requires 2.5x more backlight power (7 watts) for the same brightness than the iPad 2.
Thanks for quantifying that - the difference is 4.2 watts, which is huge. I wonder if the LED backlighting is getting any more efficient?