I agree that this isn't an apples-to-apples comparison. Delta has a ton of short haul routes in the southeast where food service isn't practical and ATL isn't that far from many of the high-traffic east coast destinations (hence limited food service on many of those relatively short flights as well), whereas AA's major DFW hub is a great distance away from all of these east coast cities and therefore meal service would be the norm on virtually all of those flights. Add to that the fact that AA does a ton of non-hub transcontinental flying where meal cost is relatively high. All of this would tend to skew costs upward, on a per passenger basis, for AA.
I would bet that, apples to apples, AA outspends Delta about a buck per passenger.