★あれ、threadStack is out of heap...
When Java creates a new thread, it pre-allocates a fixed-size block of memory for that thread's stack. By reducing the size of that memory block, you can avoid running out of memory, especially if you have lots of threads - the memory saving is the reduction in stack size times the number of threads.
The downside of doing this is that you increase the chance of a Stack Overflow error.
Note that the thread stacks are created outside of the JVM heap, so even if there's plenty of memory available in the heap, you can still fail to create a thread stack due to running out of memory (or running out of address space, as Tom Hawtin correctly points out).