Don't really get the theory behind changing the default stack size for threads. Feels like they did it just to be different which might get someone scratching their heads for a bit.
With that said, musl's current default is way too low. It's caused problems with several major applications such as git. We're in the process of trying to establish a good value for the default, which will likely end up being somewhere between 32k and 256k. I'm thinking 80k right now (96k including guard page and POSIX thread-local storage) but I would welcome evidence/data that helps make a good choice.