Quick correction: the DC power system in planes actually runs at about 15V rather than 12, which is just enough for most laptops to get by without a converter involved.

For instance, Apple sells a Magsafe Airline Adapter that will plug into both EmPower ports and the cigarette-lighter style that AA apparently uses (or used, no experience with them). It will run, but not charge, a MacBook (Pro), whose own power adapter runs at about 17V.

In this case as such, an energy-wasting AC-DC converter per laptop can be avoided, instead the plane's 400Hz AC can be converted to DC once for all outlets, and with a much smaller converter than would be required for 60Hz. Of course, a laptop requiring a larger DC voltage would need a DC-DC converter, which may negate this benefit if many of those are used. But so far as I know, the 17V used on MacBooks is actually fairly standard.

However, the 12V coming out of an actual cigarette lighter socket (as found in a car) is not enough.

As for wire gauge, the wire that actually connects to your laptop supports the same current as the DC system will, and as you may have noticed those wires aren't that thick either.

I actually prefer the DC system because of its reduced clunkyness. And with it not actually charging my laptop, it requires less energy from the plane. An AC-adapter will charge the laptop as it always does, and thus draw more current if the battery isn't at 100%.