So it's been pointed out that it's 0x10 bytes (not bits). it's 16 bytes because gcc keeps stack 16-byte aligned for x86. From GCC manual:
-mstackrealign Realign the stack at entry. On the Intel x86, the -mstackrealign option generates an alternate prologue and epilogue that realigns the run-time stack if necessary. This supports mixing legacy codes that keep 4-byte stack alignment with modern codes that keep 16-byte stack alignment for SSE compatibility. See also the attribute force_align_arg_pointer, applicable to individual functions.
-mpreferred-stack-boundary=num Attempt to keep the stack boundary aligned to a 2 raised to num byte boundary. If -mpreferred-stack-boundary is not specified, the default is 4 (16 bytes or 128 bits).