The picture is AMAZING, yes. But Highgarden is NOT anywhere near as large as King's Landing. It might be as large as the red keep. I'd believe it would take up that much AREA, but there would be huge, spacious courtyards. And gardens and stuff. The Godswoods at Winterfell is notoriously large. Winterfell is also enormous, but half of its area is its Godswoods.

I hate nitpicking such an amazing image, but i wanted to point out that the surrounding landscape should be all lush, and farmland, with pockets of beautiful timberstands of HARDWOOD trees. Not ALL forest everywhere and with no gently rolling hills of meadows and row crops.

All of the trees in this image are conifer. That's just... it would hardwood trees, like oak and maple. This is the south, and not a northern setting nor an alpine setting.

Conifer forests (pine, spruce, fir, etc.) grow in places where it's very cold (because they can withstand the cold much better), like in the north or at high elevations. Or they grow in areas with thin soils. Basically, areas where it's hard to grow stuff, because confiers are easier to grow.

But The Reach, and Highgarden are known for the opposite of that. They're known for great farmland, in a warm temperate environment. It sounds like California's central Valley, but with more natural rainfall. In other words: paradise. That fertile land with plenty of water is why The Reach is by far the most populated of the 7 kingdoms. And why The North is one of the least populated.

Dorne, being arid, is probably the least populated.