he means that the countries are so small (in area) - the size is comparable to a postage stamp (i.e. hyperbole)

It's not just that they're small in size. They're small in almost everything.

Take regulation, one of the things that's considered a measurement. Yeah, the U.S. has lots of regulations about, say, oil and gas and coal extraction. Do either Hong Kong or Singapore even have to think about oil, gas, and coal extraction? The U.S. has farms and ranches that are bigger than a few of those countries. Do those countries need to think about food regulation the way the U.S. does? That sort of thing.

For the most part they're not even in the same universe economically.

And yes, it was a bit of deliberate hyperbole.