You are correct that the final size can be estimated pretty closely by checking the sizes of the files in the application bundle. However, Apple will encrypt your binary, so in most situations your zipped application will be a bit smaller than the one on App Store.
Take a look at this : http://forum.unity3d.com/viewtopic.php?p=278022&sid=ea6e4878c70379a05c883f7093a4b41c
Tuomas Pelkonen
2010-03-13 08:19:37