Why does file size show a dash for some assets?

The file size column shows "-" (dash) for assets that don't have a local file:

• External URLs - Google Docs, YouTube links pasted in content, etc.
• Remote video media - YouTube/Vimeo added via Media Library
• External resources - Any asset hosted on another server

These assets are tracked for compliance purposes, but since there's no local file, there's no file size to report.