Besides the stack size there's also the switching overhead involved when you go through the kernel. And yes, the dynamic stack growth is an extremely important

Besides the stack size there's also the switching overhead involved when you go through the kernel. And yes, the dynamic stack growth is an extremely important part of it. Without it, you'd just have small stacks. Take an arbitrary pthreads program and restrict its stacks to a few kb. You won't have much fun. The code really needs to be written such that it can cope with small stacks. That can be tedious, so you don't want to do it for run-of-the-mill userspace code. Dynamically-sized stacks give you the best of both worlds.