Why is the size of a character sizeof('a') different in C and C++?

#include 
int main(void)
{
    printf("sizeof(char) = %zu\n", sizeof(char));
    printf("sizeof('a')  = %zu\n", sizeof('a'));
}

See https://godbolt.org/z/1eThqvMhx

When running this code in C, it prints

sizeof(char) = 1
sizeof('a')  = 4

When running this code in C++, it prints

sizeof(char) = 1
sizeof('a')  = 1

Why does the output differ between languages?
What is the size of a character in C and C++? As far as I know, the size of char is 1 byte in both C and C++.

In C, the type of a character constant like 'a' is actually an int, with size of 4 (or some other implementation-dependent value). In C++, the type is char, with size of 1. This is one of many small differences between the two languages.

Stack Overflow Q#2172943, score: 410