I have checked that my app product size is around 10MB but when I build it for archiving it shows me the Appstore size as 20MB i.e. it almost got doubled. But when I see archive in folder its size is still 10MB, I dont know why this happening may be its due to support of both architectures i.e. armv6 and armv7. If I submit the app with 20MB what my final size on the Appstore would come. If anyone have any idea then please let me know.
The iOS App store servers will encrypt the executable binary inside your app bundle for customer downloads, thus making the app much less compressible than your current archive. This is because the random looking bits that result from encryption are not anywhere near as compressible as armv6 or armv7 machine code.
So the way to get a more realistic App store download size is to remove the executable from your app bundle, zip the bundle, note the zip size, then add the uncompressed executable size.
If you have large static data inside your C code that you don't want to obfuscate, then moving that static data into a data file in the bundle might help reduce App store size.
Have you tried unzipping the ipa file (rename .ipa to .zip) and looking at the Payload? Look for unwanted files first. If you don't do a clean build, then the payload will include more junk than you intended. Also, check the size against a release build (not a debug build) because the settings could be different between the two.
Xcode now keeps your debugging symbols (.dSYM file) along with your application bundle (.ipa) so that later crash reports can be symbolicated correctly. The .dSYM file tends to be large, so your overall app bundle (.ipa) may be much larger than the application (.app) alone. Your normally only see the total .ipa size when you Archive your app. At that point, it's a pretty good estimate of how big the app will actually be on the App Store.
来源：https://stackoverflow.com/questions/11945529/app-size-double-on-archiving-iphone