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.
Apple’s iCal Team is looking to hire a software engineer. 
Luckily, you can opt to use the old interface instead by clicking on “Older Version” next to the Settings.
According to 
Aved 
An email marketer 
The poster at Aaron’s UI Design Blog 
