It's not just mAh capacity, but they drive the miliamps at twice the voltage, or more. Alkalines sag under heavy load much more than lithiums do.
Until recently power LEDs were not efficient enough to run well on one AA and still be bright. So you had to sacrifice small size, or have seriously short run times with an AA.
CR123s use a Lithium / Manganese Dioxide (Li/MnO2 ) chemistry that runs at about 3 volts, and they do have higher energy density, surge current, and better low temperature performance than simple Alkaline batteries, but they are in no way surperior to the new L91 or L92 Lithium/Iron Disulfide 1.5 volt cells when total voltage and current capability is taken into account.
Though stubby CR123's do offer a shorter form factor than AA's, they are also fatter than AA cells, and it's hard to beat the tiny profile of AAA cells in small high performance lights like the Fenix L0D.
So with the advent of, Li/FeS2 cells like the Energizer L91 AA and L92 AAA batteries, it is no longer necessary to deal with CR123's or CR2's to get the power, size, weight, and low temperature performance of Lithium batteries.
These Energizer L91 AA and L92 AAA Lithium cells offer the same high power density, low weight and higher peak current capability as CR123, and also match their low temperature advantages, and so far at least, these batteries seem to have a better safety record compared to CR-123.
CR123 cells sometimes have problems when subjected to sustained high rate discharge and there have been quite a number of incidents with these cells either venting or outright exploding in flashlights, so potential users should search the relevant threads to understand the issues. Here is one to get started with:
http://candlepowerforums.com/vb/showthread.php?t=124776
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