Why more stack are allocated by gcc in arm assembly? [duplicate]

I am trying to read the output of gcc written in ARM assembly.
The following output seems strange to me.

int succ (int *arr)
{
  return arr[0] + 1;
}

int main (int argc)
{
  int a[1] = { argc };
  return succ (a);
}

By compilation command gcc -fno-inline -O2 -S, the following assembly program is generated. I omit the out put about succ because it is not related to my question (just for stop elimination of unused variable a).

    .global main
    .type   main, %function
main:
    @ args = 0, pretend = 0, frame = 8
    @ frame_needed = 0, uses_anonymous_args = 0
    str     lr, [sp, #-4]!
    sub     sp, sp, #12
    add     r3, sp, #8
    str     r0, [r3, #-4]!
    mov     r0, r3
    bl      succ
    add     sp, sp, #12
    @ sp needed
    ldr     pc, [sp], #4
    .size   main, .-main

This main function seem to allocate 3 byte for local array a (sub sp, sp, #12). But my C program allocate just one element of int for a.
Other temporal values don't seem to use stack. Why this program require more stack volume than I expected?

My gcc version is 4.8.5. I tried in raspbian in rasberry-pi 3.

UPDATED
When I modified the length of array a to 2 (int a[2} = {argc, argc}), the volume doesn't changed (just 3 byte).
For length 3 (int a[3} = {argc, argc, argc}), 5 byte are allocated in stack.

It is curious to me.