Check out zone stacking in this link:
http://incise.org/advanced-sim-city-strategies.htmlThat guy takes it to the extreme though. I really don't see that as fun, just annoying and tedious to do. The only time I zone stack is to fit in more zones around coast lines, otherwise I leave everything whole. Some people consider it some sort of cheating. The fact is, it shows you how to do this in the manual (specifically to fit in more around coast lines).
I don't "zone stack" the airport, but I use the other trick to shrink it down. As soon as you build the airport and supply it with power, a plane will take off in a few seconds. As soon as the plane starts to take off, before it's even fully in the air, hit the plane crash button in the disasters menu and the plane will crash into the airport. Hopefully the entire airport will go up in flames. After the fire burns out, you'll be left with a much smaller airport. Hopefully only one square. The airport still functions as normal, but now takes up much less room, and pollutes way less too.
I'm gonna try another city soon and attempt 600k. I was thinking about zone stacking the power plants so they take up less room, and maybe zone stack the stadium and seaport. But I don't think that's fun.