Why is an empty binary larger than one with a variable?

Question

On my own self-learning R.E. Starting with understanding ELF and hopefully whats going on with every byte.

I build 2 simple binaries: 1 with a variable and 1 without. For some reason, the one without is larger. Why is that?

-rwxrwxrwx 1 u root 15632 Feb 19 21:30 main
-rwxrwxrwx 1 u root 15624 Feb 19 21:30 a

a.c

int main() {
    char a;
}

main.c

int main() {}

Makefile

CC=gcc
FLAGS=-O0 -fverbose-asm -no-pie

all: main a

main: main.c
    $(CC) $(FLAGS) -o $@ $^ 

a: a.c
    $(CC) $(FLAGS) -o $@ $^