Someone, who seems to have some personal knowledge of Snow Leopard, claims to have solved the mystery of Snow Leopard’s shrinking mail.app.
In a post that details the various myths 
doing the rounds on the shrinking apps—no PPC code (false!), smaller binaries (false!), missing language files (false!)—the writer spills the beans:
When you look in Mail.app you see that language files use up most of the disc space. Inside the language folder (e.g. “German.lproj”) are a lot of .nib files (the extension of Interface Builder). Inside normally are two files. One is a very small “keyedobjects.nib” and the other is very big “designable.nib” file… Now the “designable.nib” is gone. It seems like it had no reason other than to give hackers a chance to mess with the application’s UI design.
I guess he is referring to these two files which are inside each (c. 84) folder within every (18) lproj localisation folder, as in this example from the English.lproj GeneralPreferences.nib folder:
Perhaps this is as false an explanation as all the others.
Still, it has enough specifics, specifics that only someone with access to a build of Snow Leopard could know, to lend the story credence.
There used to be three files in those nibs: classes.nib, info.nib, and objects.nib.
Only objects.nib (or keyedobjects.nib) is used at runtime. Accordingly, they might as well be removed and some scrupulous developers do remove them, although most don’t bother. Apple itself seems sometimes to do so and sometimes not. In the Safari package most of the nibs in Contents/Resources have been cleaned out; most of those down a level in Contents/Resources/English.lproj haven’t been.
One wonders just how much all these would amount to over an installation. I don’t suppose it’s negligible, but I find it difficult to believe it amounts to very much.
I sceptical that this is the answer either.
FWIW, I would like it if Apple had a kind of built-in analogue of Monolingual, so that one could strip unused languages out easily. That adds up to a serious amount of wasted space on disk.
- You can install Mac OS X with the additional languages you only need.
- You can remove languages from applications using the Finder.
- On Leopard: Mail.app bundle is 290 MB on my Mac
o Number of nib “files” in Mail.app per language: 79
o Number of languages: 18 on my Mac
o Average size of the designable.nib files I looked at: 80 KB
That amounts to 110 MB. Not that bad. But I don’t know if designable.nib file are as big in general on Snow Leopard