A lot of the time that happened, when I checked it was because a lot of the assets were relatively uncompressed, so DMG-compression shrank them considerably. I haven't noticed the binaries themselves being this compressible.

But that's just "noticed", I definitely haven't paid much attention. And don't have a mac nowadays, so I can't go check my hard drive now.

I'd lean the same way as you (just a hypothesis from me too). A .app on MacOS is just a special kind of directory, so the compression covers the normal file types inside of it.

Localization files compress well, compiled code compresses well, repeating assets (@1x, 2x, 3x) and the pair of binaries in a universal app (x86_64 + arm64) do too, etc.

Ah, and dmg compression is just LZFSE, zlib or bzip2, so pretty standard stuff as far as I understand it.