It's definitely not that because he's grown so much. You shouldn't feel bad, all goldfish look very similar when they're young.Are you sure this isn't one of those things where your parents switched fish at night because they didn't want to explain that your fish died? Because that happened once only I knew it wasn't Edwina because it didn't swim to the glass and look at me like she did. It was different.
I really didn't know until I was told. I still feel really bad about not paying as much attention to them as I should have and trying to bond with them but they were fish and I felt like they wouldn't care.On the upside, I wasn't in charge of feeding them so if they died of starvation, that was mom's fault!