Applescripts `data size` command always returns 0 bytes

I tried to execute the data size command with applications, folders and files, but every time it returned 0 bytes.

tell application "Finder"
    set appPath to POSIX file "/Applications/Slack.app"
    set appSize to data size of appPath as application
    display dialog "The application size is " & appSize & " bytes."
end tell

However, if you look in details of the app it shows 444,6 МB.

Insert 'file' before the variable to get the command to work, like so:

tell application "Finder"
    set appPath to POSIX file "/Applications/TextEdit.app"
    
    data size of file appPath
end tell

However, I am not sure that it produces a correct answer.

You can also get either (or both) of the size properties.

tell application "Finder"
    set appPath to POSIX file "/Applications/TextEdit.app"
    
    -- gets all of the file's properties as a record
    properties of file appPath
    --> {class:application file, name:"TextEdit.app", index:77, displayed name:"TextEdit", name extension:"app", extension hidden:true, container:folder "Applications" of startup disk of application "Finder", disk:startup disk of application "Finder", position:{389, 1160}, desktop position:missing value, bounds:{357, 1128, 421, 1192}, kind:"Application", label index:0, locked:false, description:missing value, comment:"", size:6222681, physical size:4403200, creation date:date "Wednesday, August 3, 2016 at 10:17 ", modification date:date "Monday, July 31, 2017 at 4:47 ", icon:missing value, URL:"file:///Applications/TextEdit.app", owner:"system", group:"root", owner privileges:read write, group privileges:read only, everyones privileges:read only, file type:"APPL", creator type:"ttxt", stationery:false, product version:"", version:"", id:"com.apple.TextEdit", accepts high level events:true, has scripting terminology:true}
    
    -- after reading the above, you can see that there are two size properties to get

    size of file appPath -- logical size of the item
    --> 6222681
    
    physical size of file appPath -- actual space used by the item on disk
    --> 4403200
    
    -- odd result when using `data size`    
    data size of file appPath
    --> 260
    
end tell