How is the size of a struct with Bit Fields determined/measured?

#include <stdio.h>

typedef struct size
{
        unsigned int a:1;
        unsigned int b:31;
        unsigned int c:1;
} mystruct;

int main()
{
        mystruct a;
        printf("%d", sizeof(a));
        return 0;
}

• With int b:31, the output is 8.
• With int b:1, the output is 4.
• With int b:32, the output is 12.

Can somebody explain the reason for this?