Illustrator has a great feature called "Offset Path". When you apply it, you first enter a number in a dialog, and then it generates a new path that is perfectly parallel to the original, either larger or smaller. Scaling a path doesn't do the same thing. Here are examples:
I know Sketch can't do this natively, but I'm hoping there's a plugin out there somewhere. Searching hasn't yielded anything.
(This was my number one request when I filled out the Sketch survey a while back, btw. So very useful.)