It takes a bit of engineering to cut binary sizes down. It depends on how much they care and the cost they incur by having larger binaries. With building mobile apps you get a prebuilt XCode project and it takes a lot of effort to tinker with it to reduce binary size.

I understand that.

But how could the executables? You know, the machine code, get so big.

And I'm comparing installers with just the app!

The machine code can be so big because people put huge strings as constants. As I mentioned cutting down binary size comes from working on the code and understand how the complier translate it into machine codes. The compiler could unravel a loop, inline functions, etc... The compiler is making decisions about binary size vs speed for you. There are ways to tell it not to do that.

What does the ebay app do that it needs different binaries instead of the standard <whatever an android jar> is called?

APK? Well he's talk about iOS even then on Android you can write native code and compile to armv7 etc... and have optimized binaries for certain CPUs.

Ah, didn't realize it was iOS, didn't know iOS ran on multiple architectures for that matter. Is there a reason (other than incompetence) they don't just publish 2 versions for the different CPU's?

This is iTunes download.

When the user downloads, he downloads only the version that works with their CPU.