Swamp and forest tiles require a nearby source of water. It seems that creating a larger ocean than the range of the Ocean Giant's ability can extend the number of hydrated tiles next to the ocean.
Desert tiles require a mountain to be nearby (there doesn't even need to be a source of water on the planet). I haven't noticed an increased number of desert tiles due to having a larger mountain.
You can also create a large Desert, Forest, or Swamp region by placing two oceans/mountains far enough away from each other to not overlap.
What is the best way to create a large unbroken habitable region, and how many tiles will that region cover?