They probably didn't translate that part well when I read it in the raw I remember they said Midoriya was attacked by a villain with a quirk that make the person regress to his former younger self. And by that memory and body too since Deku come back to how he was a few years ago prior the bnka events with All might all his memory is gone since they haven't happened yet or at least the quirk made his brain think that way.
It's a bs Quirk almost like Eri'rewind but with memory manipulation on.
Don't think too much about it. It's srsly done for plot propose. I wish they didn't add the regress age technically Izuku should be 13-14 years and well.... FBI!
Why did his body change just cuz he lost his memory wtf. Good story but what was the point of making midoriya’s body younger(〜￣△￣)〜