Lucas is correct. Also, note that there is technically no way to reliably report compressed sizes of assets so that the sum of those is the total size of the compressed data.

The reason is that all data compression algorithms work by removing redundancy in the data. So, for example, if you had two assets, which are exactly the same, the compressor would be able to compress those better then compressing each individually, because it could basically just store the data once. But then, what would you report as the compressed size of each of them?

Right now, only manually. as in, make a build with something, then make a build without that something, and then count the difference.

The uncompressed sizes are a decent first step though. usually scripts and dll's compress quite a bit better than textures/models/animations.

I just had the same problem and fixed it with this little snippet:

private static void GetCompressedFileSize()
{
    string path = AssetDatabase.GetAssetPath(Selection.activeObject);
    if (!string.IsNullOrEmpty(path))
    {
        string guid = AssetDatabase.AssetPathToGUID(path);
        string p = Path.GetFullPath(Application.dataPath + "../../Library/cache/" + guid.Substring(0, 2) + "/" + guid);
        if (File.Exists(p))
        {
            var file = new FileInfo(p);
            Debug.Log("Compressed file size of " + path + ": " + file.Length + " bytes");
        }
        else
        {
            Debug.Log("No file found at: " + p);
        }
    }
}