How to modify path width?
If the artwork was just a stroked path, @Scott would be right. But from what I can see when I look at the SVG files, they are filled shapes:
See the nodes in the corners?
So you will need to Offset the path by a negative amount:
Object -> Path -> Offset Path
To end up with a thin arrow
Or, just scale down the arrow until it has the appearance of line thickness that you like, but it won't be at the original size anymore.