Zoom effect w/ certain conditions

Go to this site and scroll down to an “interactive map” of Baghdad.

I’m trying to create a similar zoom effect.
Notice that certain letters appear on the map at a certain zoom (scale) range.

I fig. it was just a MC being scaled. And when the MC reaches a certain scale % the MC within that Map MC gets visible.

Then I notice the text size does not change. So, I think my assumption is probably wrong.

Any ideas how this was done?