I'll provide the overly simplistic response to 2x2 AA vs. 2 D cell (which I'm sure will be added to, corrected, rebuked or whatever)... but anyway for what it's worth:
1x AA is 'typically' (some higher, some lower) = 2600 mAh
2x AA would be approx. = 5200 mAh
2x2 AA approx. = 10,400 mAh...