Because the earth is not flat. The earth is spherical. The earth has a shorter distance in higher latitudes. So a plane's flight is arched because higher latitudes means a shorter distances.
This is all wrong, all wrong.
Lord roo don't fuckin sleep!
So that means the world is like a cube with 4 flat sides and 2 egg shaped.
Now if you want the egg shape to scale up then it's up to you on which egg you use.
Quail, ostrage, rooster or blad eagle mixed with terradactal