このプログラムの実行結果で最後の　Size of double＊型が　４になる理由を教えて下さいな


#include <stdio.h>

int main(void)
{
int *ptr;
ptr = &value;

printf("Size of int 型 :%d\n", (unsigned)sizeof(int));
printf("Size of int *型 :%d\n", (unsigned)sizeof(int *));

printf("Size of double 型 :%d\n", (unsigned)sizeof(double));
printf("Size of double *型 :%d\n", (unsigned)sizeof(double *));

return 0;
}