Scenery artists tend to draw wide airport boundaries to “defend” their airport against overlapping objects from all kinds of default and addon sceneries. But in the long term, this does more harm than good. While there is no firm rule on what is “too close” or “too wide” for airport boundaries, airport boundaries can greatly affect scenery results after an X-Plane update or version change.
Scenery is typically only recut between versions, and the airport boundary is what is used to flatten and shape the area around an airport.
Having a tight boundary with tens or hundreds of nodes can greatly affect the detail of the surroudning base mesh. The terrain mesh uses terrain triangles to follow the airport boundary exactly, as it is not allowed to simplify (aka cut corners) here. If there are a lot of nodes in the boundary, the terrain will require a lot of triangles which reduces the detail available for the rest of the 1×1 degree tile area.
The airport boundary is NOT a tool to trace political/legal boundaries or to determine where the airport grass should be visible or not. It is to define the area where your particular layout needs to have (somewhat) flat and level ground and to be free of 3D autogen/addon scenery items. (Note that roads are not 3D items, so these are not affected by airport boundaries.)