Why did my boyfriend leave me for a FAT girl?

Were 18 and we were dating for a month and his fat ex wanted him back and he left me to go back to her. I know you all will say its whats on the inside but come on, we all cringed when we saw Zac Efron and Nikki Blonsky well this is the same. And Im a nice person. They look like Zac and Nikki together, not pretty