I've done no research here on these particular Festool batteries here, but I'm gonna give this a shot. The confusion here all comes from two weirdnesses about batteries. That, and that all that matters is power, which is affected by both voltage and current.
First, batteries do not have fixed voltages. A pack charged to 100% will have a higher voltage than a pack at 1%. I'll bet that Festool 18V battery packs are 18V *nominal*. At 100%, they are probably quite a bit higher than 18V, and much lower when drained.
Second, battery pack arrangement has a significant impact on the maximum current, both charge and discharge. This means that a 2.6Ah 18V pack is probably able to deliver a lower max current than a 6.2 Ah pack. (this is really dependent on chemistry and config, so no guarantees) This is related to the voltage sag effect [member=64076]LooseSox[/member] sees with those awesome model jets.
So, all that said, I think it's almost guaranteed the higher capacity packs are capable of higher current. Does that mean more power to your tool? Perhaps peak power, yes. But, more likely you'll notice that you can sustain the tool's usable power for longer, especially as the pack gets closer to empty. I'm an HK55 owner, and I can attest to the saw having noticeably lower power with a 5.2Ah pack at 1/3, compared to 3/3 (judging by the LED indicators).