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.
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