C assignment expression in argument to sizeof

Question

According to this question "What does an assignment return?" and What is the result of an assignment expression in C?

Sandro Munda and Ahmed Salah write

1. Assign the value 2 to i
2. Evaluate the i variable and display it

So, based on the statement above, I expect the following code will output

1
1
1

However, it outputs

1
0
1

Code:

#include <stdio.h>
void func(int a){}
int main()
{
    int var;
    //////////////////////
    var=0;
    func(var=1);
    printf("%d\n",var);
    /////////////////////
    var=0;
    sizeof(var=1);
    printf("%d\n",var);
    /////////////////////
    var=0;
    printf("",var=1);
    printf("%d\n",var);

   return 0;
}

My question is what makes this difference? Why does assignment in "sizeof()" not work as two other counterpart?