Finder may show the application bundle size. diCleaner can calculate supported related data in addition to the bundle.
Related data can include caches, containers, support files, preferences, and other associated paths. Shared containers cannot always be attributed exclusively to one app.
Permissions, disconnected disks, cloud placeholders, links, and changing files can make the total incomplete or different.
Use size as one review signal. Confirm whether the app and its data are needed, whether content is shared, and whether it can be restored.
Run a new inventory after permissions, app updates, or storage locations change.