Why does struct size changes with enum bitfield in struct (standard C), C (gcc)

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//gcc 7.4.0

#include  <stdio.h>
#include <stdint.h>
#include <stddef.h>

// OPTION A
typedef enum { A, B, C, MAX = 0xFF } my_enum;

struct my_compact_struct_option_a
{
    my_enum field1 : 8; // Forcing field to be 8 bits 
    uint8_t second_element[6];
    uint16_t third_element;
};

// OPTION B

typedef uint8_t my_type;

static const my_type AA = 0;
static const my_type BB = 1;
static const my_type CC = 2;

struct my_compact_struct_option_b
{
    my_type field1; 
    uint8_t second_element[6];
    uint16_t third_element;
};

int main(void)
{
    printf("option a    size struct       : %li\n", sizeof(struct my_compact_struct_option_a));
    printf("            offset 2nd element: %li\n", offsetof(struct my_compact_struct_option_a, second_element));
    printf("            offset 3rd element: %li\n", offsetof(struct my_compact_struct_option_a, third_element));
    printf("option b    size struct       : %li\n", sizeof(struct my_compact_struct_option_b));
    printf("            offset 2nd element: %li\n", offsetof(struct my_compact_struct_option_b, second_element));
    printf("            offset 3rd element: %li\n", offsetof(struct my_compact_struct_option_b, third_element));
    
    (void) AA; // ignore - avoids warnings
    (void) BB;
    (void) CC;
    return 0;
}

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Compilation time: 0.12 sec, absolute running time: 0.15 sec, cpu time: 0.01 sec, memory peak: 5 Mb, absolute service time: 0,31 sec