I can't speak for Chrome, but the Mac version of Firefox is a fat binary: it has both a 32-bit and 64-bit version. The 32-bit one is needed mostly for running

I can't speak for Chrome, but the Mac version of Firefox is a fat binary: it has both a 32-bit and 64-bit version. The 32-bit one is needed mostly for running some plug-ins that will only run 32-bit. On iOS obviously the whole fat binary thing isn't needed. Now half of 185MB is still a lot, of course.