Many small holes collapsing into a few large ones, perhaps? You can sometimes see where two holes merged.

That's a good idea, but if the holes collapse I expect more variation in size. My guess is that the CO2 diffuses until it finds a nearby hole.

Did someone put a whole cheese in MNR to track the holes? (I guess an ultrasound image device is cheaper. Is it possible to use a CT adding contrast to the cheese?)

Havarti has a lot of small holes, but its a different kind of cheese

Baby Swiss and Lacey Swiss are small hole varieties.

I don't believe Baby Swiss is actually a variety of Swiss (Emmental) cheese, rather than a completely different cheese. IIRC Baby Swiss was invented in America and uses a different process.

I am not familiar with Lacey Swiss so no opinion on that one.