"Definitely" was used to emphasize meaning of this sentence, right? It means 분명히, 확실히, 반드시, 물론, 완전히 in Korean. So you can say like 내가 한국에 살았으면, 나는 분명히/확실히 살이 쪘을거야. More casual? Teens may say like 내가 한국(에) 살았으면, 나 완전 살쪘을걸. Have a nice day!^^

Hello, The sentence in your title is almost perfect, you just need to change 'will' to 'would' as it is a hypothetical situation. (Second conditional) "If I lived in Korea, I would definitely get fat." Best wishes Bob