I'm a little uncertain on what you're trying to do with the document, but I believe the motion path is scaling correctly, and it might be that the other elements are more static in comparison.
This could be illustrated by adding elements at the start and end point of the sun's motion path. If you have the same pinning settings, you can see those will be at the same points.
The background doesn't scale as it isn't set to (and SVGs behave a bit differently as well anyhow). What you may want to do is create a group for the motion path animation, and set that to use a Content Overflow (in Metrics inspector) as hidden. This will clip the sun.