为什么C的sizeof的返回值类型因上下文而异？

#include <stdio.h>

int main(void)
{
    char c;

    int charSize = sizeof(c);            // Here it is an int

    printf("char: %lu\n", sizeof(c));    // Here it is an unsigned long
    printf("char: %i\n", charSize);
}
#include <stdio.h>

int main(void)
{
    char c