Tongtonghae is the term which is a bit cuter expression than fat, so you can understand the gal or boy who are curvy a bit, but not in a negative way, we say they are '통통해'. And Dongsaeng is a sibling who's younger than you, but you can use this term if you want for your close friend who is younger than you. That is an example following: 소개할게. 희연이는 내가 아는 친한 동생이야. which means 'I will introduce Heeyoen to you. She's one of my friends'. '네가 나보다 어리니까 니가 동생이잖아. 오빠라고 불러' 'you are younger than me that means you are Dongsaeng. And you'd better call me Oppa' At these cases, they can mean 'younger person who are close from talking person'

Hi, "tong-tong-hae" means "You are chubby(or fat). I think more close to chubby. it's more friendly than "fat" "Dong seng" means younger(or little) sister or brother. But I'm not sure, because "Dong Saeng" is more close and better. I think.