Why does deep recursion throw StackOverflowError rather than OutOfMemoryError?

Each method call pushes a frame onto the fixed-size per-thread JVM stack sized by -Xss, and exceeding that bound is a stack overflow, a distinct condition from running out of heap.

OutOfMemoryError signals the shared heap or Metaspace is exhausted, which is a process-wide problem. The practical implication is that you fix StackOverflowError by reducing recursion depth or converting to iteration, not by raising -Xmx.