The runway is considerably thicker than the taxiway. At least double.

Just an educated guess (experience in pouring concrete). The taxiway looks to be 6", the runway is 12"+ with larger, and more, re-bar. Probably more like 18". Remember, you've got to allow for a million pounds to slam down on it with incredibly high point-loads.

That being said, if a large enough sink hole were to form under a runway, it could fail. However, a sinkhole would be less likely to form since, due to the thickness and fewer joints, water infiltration would be a lot less than on a taxiway.

I'd say, due the fact that this is the first one (ever?), there's nothing to worry about.