[quote=ArneyAt a reasonable 250mA current draw, that single D cell will last 16 hours. But the AA will last 9 hours, so the D cell provides less than twice the runtime even though it contains 6 or 7 times the total energy as the AA cell. Now which one looks more efficient? [/quote]

Can not agree, both suppliers data and experience suggests that a premium make alkaline D cell will supply 250ma for at least 50 hours and perhaps a bit longer.
A similar make of AA cell might last 9 hours, but 7 or 8 hours is more likely.

I would expect a zinc carbon D cell to last about 16 hours at 250ma.